Tim Flagler put together this neat video showing some of the aquatic life forms that trout feed on in March and April here in the Northeast, and the flies that imitate them.
All of these flies are available from your local fly shop, or if you are a fly tyer, you can follow the instructions here - the flies are listed in the same order they are shown on the video. Just click on the name and the video will load:Simple Scud
Chimarra Caddis Larva
Little Black Stonefly Nymph
Vinnie's Isonychia Nymph
Gold-ribbed Hare's Ear Nymph - Beadhead
Soft Hackle Streamer
Tie some up, fish them hard, and sharpen your hooks!