Fall is one of the best times of year to toss streamers. Streamers make up a broad category of flies which commonly imitate minnows, leeches, crayfish, or some sort of intrusive conglomeration mounted on a hook or hooks that really aggravate the fish. Brown Trout are fall spawners and some Rainbows will also spawn in the fall (most Rainbows are spring spawners, but some of the hatchery mutuant Rainbows, mostly sterile, will attempt to spawn in the fall as well). As Trout prepare to spawn they become very aggressive and territorial willing to chase, smack, and eat anything invading their personal space. Fall often brings some of the biggest river Trout out to play.
Streamer fishing can be as basic as cast and strip, but concentrating on some of the little details will bring more fish to the net.
First, casting a weighted fly and or sink tip fly line can be a chore. Work on shooting line rather than carrying line. This will lessen the number of times you impale yourself with a #2 hook. Accuracy is VERY important. Many fish are near the bank. Cast as close to the bank or shallow edge as possible and get in contact with your fly ASAP. Again, many fish are taken during the first few strips of line.
Second, keep your rod tip low. I usually like the rod tip touching the waters' surface. This eliminates slack, and provides better strike detection and quicker hook sets. Also of note, set the hook with the line, not the rod. When you feel a strike pull with the stripping hand. For a right handed caster you'll be retrieving the fly with your left hand, when a strike is felt pull the line tight with the left hand. This way if the fish misses the fly you have only moved the fly a short distance and the fish can easily come back for a second take. If you set with the rod you will have moved the fly a much greater distance and are also out of contact and position to set again in case of a second take. Once the hook is set with the fly line, then lift the rod and fight the fish.
Third, play with different retrieves. Some days they like it fast, others slow. Often times a few quick pulls followed by a stop is the preferred pattern. Letting the current belly the fly line and swinging the streamer across the stream can also be effective. As a general rule the colder the water the slower the retrieve, the warmer the water the more the Trout like it moving. That said, fishing isn't an exact science and each day has it's own mood. Try changing retrieves until you settle on the day's best presentation.
Fourth, fish heavy tippet. 3X is as light as I go with streamers. The straight line connection (very little slack) and voracious takes will often leave you less one fly, and the fish will have a hood ornament until he can shake your streamer loose. Usually I go with 2X unless I'm casting streamers larger than a #2, then adapt as necessary. My favorite tippet for streamers is Umpqua Superfluoro. In 2X it boasts an 11 pound breaking strength and is dense (that is a good thing) and abrasion resistant.
Fifth, cover a lot of water. When fishing streamers you are looking for the aggressive fish. Standing in one place repeatedly shows your fly to disinterested fish. Move, cover new water and show your fly to new faces.
Finally, there are thousands of streamers available. Talk to your local experts and see what has worked for them or do a little discovery mission on your favorite waterway to find out what kind of inhabitants could be imitated with a streamer. Sometimes the best flies imitate nothing. In addition to imitative patterns, try wild colors or bulky flies that push a lot of water. Below are a couple of "seasonal" streamers. A Halloween Bugger and an Autumn Splendor. I've taken many fish in the fall on both of these patterns.
Directions on how to tie the Autumn Splendor can be found here: