A trout takes a fly faster than you can blink. The hook-set is what turns a take into a fish. Match the technique to what you're fishing.
Dry fly. When you see a rise on your fly, lift the rod tip up and slightly back. Smooth, not sharp — a yank tears the fly out of the fish's mouth. Many beginners blow takes by trying to set "harder." Steady is right.
Nymph under an indicator. Anything that isn't the indicator's normal drift is a strike. A pause, a dip, a twitch, a sideways skate — all of it. Lift the rod tip the moment the indicator does anything unusual. You'll set on rocks ten times for every fish; that's correct, not a problem.
Streamer. Set sideways with a strip-set: instead of lifting the rod, pull hard on the line with your line hand while keeping the rod low. Trout often hit streamers on the strip itself, so you're already in position.
After the set: keep the rod bent (tip up, around 11 o'clock), let the reel drag absorb runs, and walk the fish to slower water to land it. Don't horse it — the leader breaks before the hook bends.
A trout that throws the fly was usually never hooked properly. Faster doesn't fix that — only tighter line and a smooth, committed lift do.