I agree that the flinch is associated with your anticipation of the shot. Try to use less focus on the shot and more focus on the release. Aim the bow, then concentrate on releasing. You should be surpised by the release. When you are releasing correctly, you hand should jump back after the shot, not go forward, or stand still. When this happens, everything is natural. You won't have to worry about flinching because the shot will already be over. Phil hit the nail on the head.