When fly fishing for trout, on river or stillwater there are a variety of techniques which can be used:

Using different flies

Lure stripping (Streamers)

Here the intention is to mimic a small fish or fairly rapidly moving bait. This technique has been known to solicit disapproving comments as though it were 'unsporting', but personally I find it has produced some of my best fish - and it does keep the angler active! A fast sinking line, cast a prodigious distance, using a weighted fly (such as a leadhead or dog-nobbler) brought in with a series of fast jerks is the usual methodology. Sometimes the rod is placed under the arm, so that both hands can be deployed to bring in the line at the fastest possible rate.The fast sinking line is used to get down to the bottom feeders, which tend to be the bigger fish, and the retrieve action produces a 'skittish' movement of the lure which is similar to that of a fry.

           Dry Fly - moving

There are some flies such as the Popper, Muddler, and Hugh Falkus' Surface Lure, which are designed to attract fish by the action of creating a surface disturbance. A Muddler retrieved upwind of a rise can often result in a bulge in the water as the fish follows the fly. There is often great indecision on the part of the angler as to whether to speed up or slow down when this happens. I tend to speed up and make the fish feel that it may lose a meal if it does not strike soon!

Dry Fly - static

Here a fully formed winged insect is imitated. Many consider this to be the highest form of fly fishing, and dry fly anglers are generally revered as having mastered a difficult technique. A floating line, but with a de-greased leader, using a floatant treated fly (such as a Carrot Fly) which is usually left static, or to drift with the current or wind. Throughout, however, the angler stays 'in touch' with the fly by maintaining as straight a line connection as is possible and if necessary by retrieving line. The excitement with this technique is that the angler actually witnesses the 'take'. But intense concentration is required, generally all that is seen is a flash of the underbelly as the trout takes the fly. General advice is to pause before the 'strike' but personally I strike immediately - perhaps my reflexes are slow! I find that on the dry fly, if I feel a 'tug' before striking, it is too late and the fish will have already discarded the sharp object in its mouth.

Surface film fly

There are a series of popular new emerger flies (such as the Shipmans) that are fished 'in the surface film' rather than on it . These represent the nymph in the process of metamorphosis (or in the case of the Shipmans - the discarded shuck). Again these are often fished static.

Wet Fly

A technique that has stood the test of time, where the fly (such as the Invicta) is fished sub-surface, and is retrieved slowly using a 'Figure-of-Eight' manipulation of the line in the hand. A floating or intermediate line can be used.

Nymphing

A deeply fished fly (such as a Caddis or Stick Fly) is inched very slowly across the bottom, using a 'figure of eight' retrieve. The orders of the day are long leaders and patience! A floating line is used, and the line is watched closely for the slightest perturbation.

Fishing the Booby

A 'Booby' is a very buoyant fly. It is usually constructed using two small balls of polystyrene held together in a piece of nylon stocking at the head, and some flowing marabou at the tail which gives some movement. The fly is attached to a short leader of about 12 inches, on a sinking line. The line is cast out as far as possible and left to sink to the bottom with the fly moving gently in the lower currents just off the bottom. The angler simply places the rod securely, and waits for a take.

Using different Fly lines

Floating and weighted flies and lines can search all the regions between the surface and the lake bed. The floating line is easier to control and offers visible indication of the 'take'. Using a count-down before retrieval with a sinking line, repeatable depths can be explored for feeding fish. The higher density (sinking) lines reduce air resistance when casting, and result in longer casts.

Lough Style

Short, overhead casts using a floating line from a sitting position on a boat, on a lake. The line is roll-cast forward to bring the fly to the surface, immediately followed by a back cast and then into a single forward cast. The line is retrieved at about the rate of drift until the cast is fished out, when a new cast is made. Fan casts ensure that the maximum area of water is covered.

Stillwater Bank Fishing

For a right-handed caster, a left-to-right wind is preferred, so that a gust is liable to move the fly away from the angler. The leeward shore is usually more productive, but places a premium on casting ability.
A 'fan' cast is employed to cover the water. Line is retrieved or 'mended' in order to maintain a straight line between the fly and the angler. The rod points down towards the water and the line between the tip ring and the water is often used in the same way as a 'swing-tip' in coarse fishing. On detecting a bite the rod is raised from a downward position to almost vertical. This rapid recovery of line (or 'strike') sets the hook. The rod now acts as a 'spring' to cushion the lunges of the fish. Line is forfeit when it is deemed that the fish is putting undue pressure on the equipment, but during this 'playing' phase, the intention is to tire the fish and bring the fish near to the angler and the net.
Eventually the net is placed in the water and the fish is brought over it. The net is then raised and the fish is finally captured.

Fly Fishing for Sea Trout

The classic text to refer to on this subject is the book by Hugh Falkus. A masterpiece and worth the room on any fisherman's shelf, it gives all the information and more that you need to catch these elusive fish.
In general I tend to follow his advice concentrating on dusk as the most likely catching time, but I do not fish through the night as he suggests. I am not particularly enamoured with Bruce & Walker rods which he recommends (rather slow-actioned) and find that a 10ft trout rod is fine, rather than the 10ft 6in rod which he commends. Perhaps the most important aspects are:

1. Do not start fishing until it is dark, so that you do not put the fish down straight away. Wait until you hear them start splashing, and make plans for how to fish that spot.
2. You must be at ease:
   with your equipment (rods and lines for casting, know instinctively where all your accessories are, and wear adequate warm clothing)
   with the river (so that you know the terrain exactly and can wade confidently, and are ready to beach or net the fish easily)
   mentally (so that you can concentrate 100% on the fishing and not be concerned about bumps and groans in the night - there is often a lot of wildlife activity at night).
3. Concentrate on where you expect the fish to be (neck of a pool) or where there is a lot of activity.
4. Keep everything simple. Use a single fly and a short leader as trying to sort out tackle problems at night can be difficult.

Salmon fly fishing is distinctly different to trout stillwater fly fishing:

• The flies are usually heavier (doubles or trebles)
• Position is critical to success. You MUST be fishing over a place where salmon prefer to rest, unless you see a moving fish. If local advice is not available to point out the lies, then I would make for 'deep runs' using a sinking line (or at least a sink-tip line).
  
• It is lure fishing rather than imitative of insects (ie the fly should be moving as though it were a small fish).
• As you must tackle-up in expectation of hooking a large fish, the leader strength etc must be suitable for this eventuality. I normally fish with a 16lb breaking strain leader.
• In order to get the fly down, in moving water a sink-tip flyline is almost essential. These have a poor name with stillwater anglers as they are reputed to be awkward to cast. I use a fast-sinking braided leader on a floating line, which gets round most of the problems.
• In order to stay 'in touch' with the fly and to avoid rapid accelerations of the fly which would be uncharacteristic of a small bait fish, it is necessary to 'mend' the line occasionally. This involves 'flipping' the line so that the downstream belly created by the action of the current, is converted into an upstream belly. This action enables the angler to fish for the most part with a fairly straight, direct connection to the fly.
• On a spate river, the most productive time to fish is as the river level falls after rain and the water starts to clear (fine down). That is why a river level indicator is so assiduously consulted by the keenest anglers. Dawn and dusk should definitely be fished, but you may catch fish during the day (and also at night).
• The general advice on connecting with a salmon is to let the fish take line, and then tighten into the fish without a deliberate 'strike'. Personally, I have both let the fish take line (a lot) and also 'struck' immediately without losing the fish. A trout fisher's natural reaction is to strike, but perhaps this instinct can be curbed when the prize is so much greater.
• When playing a salmon you are advised to move downstream, so that the fish has to contend not only with the pressure of the bent rod, but also is forced to expend effort against the river current.