The best fly I have ever used for crappie is Al Campbells Crappie Candy. It's in the Fly Of The Week Archives. Just type it in the search box. The next best one I have used is the Electric Chicken:
After this, any smaller clouser, streamer or bugger will work. I'd recommend chatruese, and white for daytime use, and black for nightime. Use brighter colors in stained, or darker waters, and darker colors in clearer water. If you are catching a lot of small ones, move a little deeper.
During pre-spawn, look for schools of staging crappie in structure that is close to suitable spawning areas (flat shallows with nearby cover). Use a sink tip to fish in about 7-15 feet of water. During the spawn, just throw anything at them in shallow flats. It doesn't really matter what. Fish anywhere from 6" to 4 feet deep.
After the spawn....fish for something else. Crappie are moody and suspend, anywhere, without regard for structure, the thermocline or anything else. They can be anywhere, from 10' to 100' deep. They will refuse to hit anything unless it is almost placed in their mouths. Summer crappie are tough. In fall, look for them in 5-15' of water around structure. They bite actively, stoking up for the winter. In winter, they will hold in 15-20' of water, and hit very light and slow. This is not a good time for fly fishing, as far as crappie are concerned.