One highly effect way of catching fish on the fly is nymphing.  Its a well known fact that the majority of a fishes diet lives below the surface of the water.  Using these techniques, you can catch trout in streams all year round by matching your nymph fly to local bug species.  When trout and salmon begin to enter their natal streams in the spring and fall, you can be sure that these fish can be taken when nymphing.  Try it out with an egg fly for those migratory fish!