From speaking with Lefty Kreh at the Virginia Fly Fishing Festival this year, I was given several good pointers on fly fishing for crappie. One, in particular, I found extremely effective, both in my fly fishing for them and traditional crappie fishing.
When the fish are difficult to pattern and you luck upon a school of crappie, it is essential to stay with the school as they move to consistently catch fish. The way Lefty has done it over the years is extremely simple. Take a balloon, a size 10 hook, and a length of red string. Once you catch the first fish, inflate the balloon, knot it with the thread and tie on the hook. Hook the fish through the mouth and release it back into the water. The balloon will move freely while the fish moves with the school.
The red thread is helpful in that it indicates which way the fish is facing, showing you how to present your fly or lure to the fish. If the fish is facing into the wind or current, go upstream of the fish and make you presentation look like a minnow being swept down to the fish.
Following the school will keep you into fish all day. The last piece of advice I was given sounds perfectly reasonable: if you are keeping a mess of fish for dinner, you always release the one attached to the balloon because he was your helper all day. Next time you are having difficulty targeting crappie, try this technique. At least it will keep you casting to the fish rather than searching for suspended fish in what appears to be featureless water.