Cardinal Sin 2 - Don't correct shooting problems on the shot. This makes players tentative on the shot, whereas effective shooting is natural, no thinking is involved. Make sure the fingers are spread apart as far as possible and the hand is shaped like a claw. Do this with either hand. Flick wrist or wrist work - Shots are flicks of the wrist, so the wrist must be loose.
Stiff wrists are the most common problem for high school and up male shooters.
One exercise involves just loosening the wrists. Wring or shake the wrists out till they are looser. Another exercise involves flicking the wrists. Flick back and let the wrists come forward naturally. Try doing this with the arms overhead, elbows straight.