We did some training with Clint Moore a few months back, and snapping was a big part.
Set up behind a bunker (we used a small cake- train in the hardest bunker and the bigger bunkers will feel easy!), and shoot at a cone, or pole, behind another bunker a sensible distance away. Start slow, and try and hit every time, preferably with someone watching and correcting mistakes. 5-10 shots from left, right and above, trying to hit within 5. If someone doesn't hit all three in those shots, punishment lap of the field! Makes it more realistic if there is some pressure on each time. move the target further, or tighter behind the bunker to up the challenge, or lower the number of shots for success. Also try rattling off ten very quick shots each side of the bunker, and then starting to count the shots to simulate being tired/stressed as in a game. Finish with some one on ones to put it in context - only shoot it you see you opponent to ensure they are playing tight.