Drop Shot Fishing
A Technique that still works after your retrieve makes presentation "go vertical"

Dropshotting is effective on deep, suspended hard to catch uninterested fish.  It is a technique which originated in salt water jigging. However it has developed into a very sensitive way of fishing from a boat or float tube. Specialised hooks and weights are now available for drop shot fishing which make this technique easier and even more effective. In the US many major rod manufacturers have designed tapers specifically for the technique. For ourselves any medium-through action spinning rod will do fine. If you must use a fast action rod then the shorter the better.

On Irish loughs, during early season the method most seen is trolling, usually Rapalas, but other techniques can work too. If you've located large trout (see left) hovering or cruising slowly over deeper structure like eg last years dead weed beds, position the boat or tube over or near them and drop the lure or fly right on top of them.
Nod the rod tip gently with small rise-fall movements, alternating into vibrating movements. This shakes the fly/bait, still keeping the weight on the bottom. Take your time as if you were using a live lobworm or live fish bait, and be happy to give the predator time to make it's mind up, or get teased into action. Take long pauses of totally static motionless presentation. Tremble the rod tip with no other movement. Fish slowly and carefully. Especially in the cold early months like March and April, this works well.  It is excellent when the dog days of summer push the fish deep.
Only trolling a sliding float gets similar results (ultra slow presentation at a controlled depth) to drop shot fishing, and DS is more versatile, since it can do a fast presentation too, as well as cast to greater distances.
If you are used to reeling in to make lure lifelike it is difficult to take on the idea that a static artificial lure is lifelike enough to fool predators. This video clip is here to show you just how real they look and move under their own very light weight while the lead is resting on the bottom.      

Rod and Reel
Through action rods and fixed spool reels are best. I use a through action 7' spinning rod for trout and perch, and for pike a stronger 10' MkIV S/U 2.25lb TC carp rod with sliding fittings, and the reel mounted low (for seated tubing convenience). A boat angler would prefer the normal reel fittings/position. 
For reel choice multipliers are not free running enough to drop a light almost weightless bait, at natural speed. So a fixed spool spinning reel is better for DS fishing for trout and perch. A multiplier can be used for pike and large trout with a little practise, due to the heavier baits, but the fixed spool is still better there too.
I use ABU Cardinal 3 and 4 series for light, and a Shimano 5 series Baitrunner for heavy stuff.
To be honest, a multiplier is more suitable for float tubing, because the reel, being on top of the rod is not touching the apron table top we have in our seated position, and the handle has more clearance as a result. Some day I will get one of the latest compact super multipliers, with small low inertia spool, and give it a go for this.

Tying the Hooks - Knots for DS
Tying the hooks on drop-shots is a refined technique, and can be done a couple of ways. The usual advice is to use a Palomar knot, beginning the knot with the hook positioned point side up. This is done before tying the sinker on. This is done so that the hook lays at a right angle to the leader. This is a better way to get a good hook set on light takes. Another way can be to take the leader end, after the hook is tied, and thread it back from top side downwards through the hook eye, then attach the rig lead. This way the hook shank lays horizontal sticking out from the line, which improves hook ups.

The special requirement of a drop shot rig for the hook to stand out has led to many anglers to using the Palomar knot even though it is not particularly good with the light line of a typical drop shot rig.  The Palomar is a good knot for thick line designed for big game sea fishing, but it tends to cut itself and rapidly weaken in use with light/thin lines. In thick lines it's a 95% strength, but in the 4 - 8lb breaking strain lines it is unsafe in my opinion. Tight wraps over small round hook eyes and snaps tend to fracture the surface of tightly wrapped line. As a result thin lines seem to weaken much more than thick ones. Fluorocarbon tends to have a stronger surface than mono and lessen this tendency to break at the knot, but it is always a good idea to re-tie knots in fluorocarbon after catching a heavy fish, or having a stress tension from pulling out of a snag.

My favourite knot, the Uni / Grinner / Duncan Loop Knot is a lot stronger than the Palomar, and is I believe the best standard fishing knot for general purposes. Unfortunately it is not suitable for dropshotting because the tag end is facing back up the line, and not in a direction that easily lets it be threaded back through hook eye, downwards to the lead sinker. Use the Uni knot to tie hooks and swivels on in all other circumstances and you will have a reliable knot. The double Uni (not shown) is the best line - to - line joining knot I know.

A Double-line Clinch Knot is also stronger than the Palomar and it will make your hook stand out properly, so it's my No 1 knot for drop shotting. It should be completed with a doubled section of line. This knot has the advantage that the line tag end comes out at an angle which is conducive to being threaded through the hook eye without having to make a line weakening 180 degree turn. Like the Uni/Grinner/Duncan, it is also a 95%-100% line strength knot, but the tag end exit (near the hook eye) and it's direction are more suitable for DS fishing.

When the knot is completed, moisten, and draw tight. Then cut one of the double tag ends at the knot leaving 2mm sticking out. Unfold the tag loop, and your other tag end doubles in length to become the leader down to the sinker. Insert the two foot tag end leader down through the "top" of the hook eye so the hook point faces up. The weight will go at the end of the tag end.

There is a new hook I have not yet tried out (shown left). They are the TTI stand-out hooks which are designed to hang correctly for DS no matter what. They look good. They are of course designed for bass fishing in the US, and that means that large perch over here with a similar soft mouth structure will be hooked just fine on these hooks. I have not yet figured how these hooks will work in our local hard mouth predators like trout and pike. I will test some out soon and report.

Sinker Weights for Drop Shot Fishing
You can use almost any kind of lead/tungsten, but I like the "quick release" drop shot weights. If the conditions on the water change, such as the wind picking up, the current increasing, or if you move to deeper water, you can quickly change to a heavier weight without having to retie.  The weights are specifically tailored for drop-shotting techniques. The best are long and thin to slip through weeds, but some are ball shaped, as has a swivel-like line tie that reduces line twist. Line twist can sometimes be a problem with these rigs in wind, or deep water situations, and anything that helps reduce this is a definite plus.

This type of weight also has something the others don't. It has a line clip that lets you change the distance between the lure and the weight, without having to retie. 
Another technique for drop-shotting, is to tie a regular lead head jig, (usually a 1/4 to 3/4 of an ounce), at the leader end instead of the lead weight. If you do this don't use the Palomar knot as it is not strong when pulling from the tag (sinker) end, and a fish hooked on the jig will exert it's  pull from there.
Some anglers use a "pinch-on" split shot or two for the lead. This is very handy, easy to adjust, slips off the line when snagged. I assume this is how Drop Shot fishing technique got it's name.
For a heavier rig you can also thread a bored bullet weight on the drop-shot leader, below the hook and lure, with a split shot squeezed on below the bullet weight to hold it in place. More weight can easily be added to this rig quickly, and you can spend more time fishing.

A trick I use in rocky places is heavy lead wire, wound round a nail to make a thin tube, and threaded onto the line with a No 1 to 4 or BB shot below to lock it on. See photo. This is a fine snag resistant set up, and if it happens to lock into a crack between rocks, the shot will slide off releasing the wire and allowing recovery of the flies or bait.

It depends on wind, drift speed, but as a guideline from shallows to 20' of water use 3/16 oz, for 30' deep consider 1/4 oz as standard, and for deeper 30 - 50' try 5/16 oz up to say 1/2 oz.

Swivels and Line Type
I tie on a swivel as a connection between the line and leader to reduce line twist. I always use a black swivel for this and other techniques in clearer water, if the BB swivels are too large use two smaller standard swivels separated by 1' of line.
I select the smallest swivel I can. Trout and perch feeding on nymphs will always see the swivel first if it is the same size as their food, and has movement, and this is to be avoided like the plague. For this reason be willing to consider that a swivel might reduce the number of takes in certain conditions, and go without if necessary.

Most anglers recommend a braid as the non stretch quality aids in detecting subtle strikes in deeper water. For very deep water (which is a rare fishing occasion) I admit a braided line won't be seen in peat coloured water, but it sticks out to much in normal depths (under 30 feet) and clear water areas, I use a Fluorocarbon line, partly because it's harder for the fish to see than the braids, this is important.
Also, by using less visible fluorocarbon line, I can go up in size to a higher pound test without the fish being able to detect it.

Finally, braids show up on the sonar all the way from top to bottom, and this really clutters up the screen, at a time when I'm supposed to be fishing for difficult fish that are in a neutral state at best, or not feeding at worst.

My favourite line is tried and tested, abuse taking, Maxima green mono in 4lbs and 6lbs strengths. For pike I use Maxima and Berkeley in 12lbs and 15lbs from reel to wire trace, with wire from main line down to the hook, and Maxima/Berkeley from hook down to sinker.
To give an idea of how I weigh up the various factors involved - if I fished crystal clear waters frequently, I might use fluorocarbon more. If I fished deep ALL the time, down where light levels are lower, I would consider braid, or heavy fluorocarbon, as their low stretch gives good "remote feel awareness".
For pike no special requirement to reduce line diameter arises, since the strengths above work fine in all conditions.
For trout in clear water and educated by being released a few times, Maxima 3lbs is invisible (on the surface in my own tests on educated trout in daylight), and clear fluoro of 5lbs (probably working strength 4lbs after knots) is also invisible. Heavier or darker lines near the surface can be seen and in bright light reduce the takes received. Under the surface 6lbs Maxima and 8lbs fluoro produce no reduction in the number of offers. For rough water, overcast conditions, and night fishing use heavier because you can, and it's better, I use Maxima 6lbs for small nymphs and 8lbs for leeches and lures.

There is an advantage of having a longer length which anglers new to drop shot fishing might not think of. I am referring to the feature whereby your bait gets dragged down by the weight at speed, but after the weight lands on the bottom, the bait stops being pulled down, and begins falling more slowly under it's own weight, and then it is under the control of you and your rod.
Now (with the sinker on the bottom) you can hold the bait while you tighten up slack line. But alternatively you can choose to give your bait a weightless ultra slow free-fall during which it descends a bit, then you twitch it back up until the line tightens to the sinker. This is a particularly deadly moment after the cast.

The ability to free fall your lure is best if you are fishing vertically, with no wind, wave, or current drag pulling on a bow of slack line. If you are casting away from your position, these wind, wave and drift will place a certain minimum tautness in the line, which is difficult to overcome. The dry fly fisherman's trick of "mending line" is how you get the bait to sink downwards, in essence feeding extra line into the line "bow" so drift takes the line from your end only, thereby allowing the other end to remain free of tension.

On a clean bottom with bottom feeding fish, reduce the sinker leader to between 9" and 18". But with more active fish a higher bait is more visible and will get more takes by being seen by more fish. So in these circumstances lengthen the sinker leader from 12" to 24". The leader can also be lengthened if there is a bottom covering layer of soft weed like chara which fish like to pass over while hunting for little food items that graze on the weed.
Be sensitive to changes in line angle caused by casting, or drifting off the bait. That reduces the height of presentation and might create a need to lengthen the leader.

During the early season it is common to find non-feeding trout or pike in mid water, cruising over deeper features like dead weedbed root patches from the last season. Midwater predators are usually not feeding, so they're hard to catch by any method. But if there were to be nymph activity, they tend to be located already in the place where it will occur, like suspended over last years gravel bed (stick insect - sedge pupa), or dying weedbeds (asellus water louse, gammarus shrimp). This is a good time for an extended leader to achieve mid water presentation, with the weightless trembling at the level of the fish.

HOW: Drop Shot Fishing Technique
If you are in shallow water, where fish under the tube will be spooked by it's shadow or presence, cast away to open water, or to a fish holding feature.  Cast the rig anywhere you want.

For illustration, have a look at the action of a neutral density Roboworm in this video clip:
                           

If you are over deeper water, you can either cast or if fish are on your sonar at the time just lower it down vertically.
If you're new to DS fishing lower your rod top occasionally and watch for slack in your line. This shows the weight is resting on the bottom, and is where it should be. If the line stays taut it means the weight is still falling through the water and needs more time to reach bottom. While the weight is sinking, keep the bale arm open so line can spill off the spool and allow it to crop vertically, instead of making a pendulum swing in towards you as it sinks.
After your weight touches bottom, gently vibrate the rod top, and by extension, the soft plastic lure and make it twitch and hop a few tiny movements off the bottom while the weight stays on the bottom. If you made a cast, gently retrieve the bow in the line but try not to disturb the weight, other than the smallest amount. Fish have time to inspect it, and the tinier the movements, the more lifelike it is. They tend to just suck it in and proceed on their way.

Be prepared for extremely gentle takes
The fish are taking a slow moving prey at their own level. They did not chase it and build up speed prior to the take. They also did not rise greatly in the water before the take, so they will not make a turn and rapid descent after taking it. For these reasons, and a couple of others the take can often be tiny, almost undetectable in fact.  Often, on a subsequent twitch or draw you notice a little extra weight or inertia on the line. After a lift of the sinker to a location closer you may notice that the expected duration in time for it to touchdown and create a slack line at the rod top has been mysteriously extended.

Either way, lift in, do not strike, just lift as it to make a retrieve. The fish should be hooked in the top lip.  If you strike into a big fish, while it's body is at 90 degrees to your direction of pull, and also on a short line, you will most likely either break the line, or rip the hook out, or cause a shallow hook hold which gives later on in the fight that is now about to begin. the vertical presentation has advantage after hooking. If a big fish head shakes, a tackle wrenching force when playing it from the side, there is no vicious tugs to a line pulling upwards. So a big fish is denied one of it's more potent weapons for a part of the fight, until after it runs away and the angler is no longer above.

After a while you will learn to gently make a slightly bigger lift (of the sinker) than usual, every so often, so as to see if a fish is munching on your soft bait. It's soft so they will munch for a while, not dropping it after the initial crunch. Trout that do drop it after the initial crunch will circle around and nail it again so there is not much hurry to strike. Wait until you feel that the fish is holding the bait in it's mouth at that precise moment, before taking action.

Using a small bait, helps ensure that the fish is not holding the tail of the bait, predators naturally take it by the area of greatest mass, that is, the chest, shoulder, and head. If the bait is excessively small, and they want it they will engulf it all in one go, but quite likely without making a big giveaway tug on the line.

Fly, Fry-lures, & Dead-bait Minnow all work with this
Yes you can fish flies, soft lures and real baits ALL with the same technique! For this sensitive method of fishing smaller lures and baits are appropriate. Flies and nymphs are good, and mayfly, corixa, water louse, shrimp, or damselfly nymphs work well. Streamer flies can work, and I prefer marabou which adds life into a larger size nymph. On the whole as it gets bigger, I want it to be made of more mobile materials, so I look to the rubber and plastic lures when it's over 2" or so.

Flies are so small and light that a short dropper can sometimes improve presentation over that produced by the "off the main line" normal setup. In other words a traditional wet fly leader with two droppers and a slim weight where the "tail fly" normally goes. A snaggy bottom can sometimes be fished very effectively with the flies rather tighter to the bottom rocks than a conventional sinking fly line would allow. This is shown in the picture right.

Fin-S shad, plastic minnows, and tubes with the multi legs skirt are good in smaller sizes for trout and perch. The larger 4 - 6" sizes are excellent for pike and I guess ferox trout too, though I haven't proven much there so far. Most trout I've taken were on minnow and perch/roach fry marabou fly-lures. I have used frozen minnows with a lot of success.

If you use a longer "bass type worm" think about how it will suspend as it moves very slowly.
If the plastic is heavier in density than water it will droop, hanging down and not look natural. It will depend on a cast and retrieve to maintain it's horizontal status, and not fish right when directly below.
The best plastic lures are neutral density, the same density as water. When nose hooked these ones will conveniently hang horizontal, and not droop head up-tail downwards as some other heavier ones do.
Test your lures at the surface layers, observe them in visible shallows, and learn how they work, or don't work, and what a jerk and a tremble makes them do when down out of sight. What is good for ordinary spinning may not work drop shotting.

As always in fishing, fish, experiment and find out, and enjoy the experience.