My team has used quite a few over the years. One thing that we have done to stress the importance of communication is to play 3 on 3. Each side would have two players up front, and one player in a cover position. The cover guy would not have his marker. This would be to stress that a player can be a factor with only his mind and his mouth, giving his front guys information.
We have done many snap-shooting drills. Most are 1 on 1, in two different bunkers that are between 20 and 50 feet away. No more that 3 balls per string, and sometimes a cover player is put in just to keep the guys performing the drill from hanging out. The point is to get out, shoot quickly and on target, and get back into cover. We have also added extra targets like gatorade bottles to shoot before you can shoot at the other player.
Here is a new one that I want to try this weekend. You have a 3 on 3 situation with one cover guy per squad. The would also be about six target (three per side) that must be shot. The whole thing would be for the back player to coordinate movement so that his front guys can shoot the targets, and shoot the other players. All players are live. So, in total, each team would have six targets to eliminate (three bottles and three players) before the game is done. I want to try this to keep the snap-shooting sharp, coordinate some movement, allow trust to build between the front guys and the cover guys. I may even put an empty paint box out there to be bunkered. Perhaps before two of the targets can be seen. I don't want to just put them on top of bunkers, but rather position them like you have to get an angle on a player.
We have done many 3 on 1's. This helps the 3 players to coordinate and finish the game (put a time limit on the drill) and also gets the single player plenty of experience getting pounded on and snapshooting under a lot of pressure. No doubt that you will benefit from this drill in a tournament if you are the last guy in. This one has worked so well for us, that we have one guy that can take down at least two when he is getting rushed by 3 or more. Hell, he has even turned them around and won games for us. Our other guys have improved on this one as well, but with the one guy, it is a sure bet that he isn't walking to the deadbox alone.
Have your players practice shooting with boths hands, from all different positions and practice in as many different style of bunkers as possible. If you play hyperball, airball or some other speedball all of the time, then when you travel and get in the woods (like the Zap Am Open) you'll be stuck in the woods without much knowledge of how to do it. Getting back to the older way of playing now and then will keep things sharp.
This isn't really a drill, but go to as many fields as you can and walk the fields a bit before you play them. Try to figure out the damage spots on the field and how to get your guys into them. Field walking is super important. You only get a few games to prove that you belong in the next round. Spend some time walking the field. Get the players to sit in the bunkers like they were playing them. Have your players stand on the other side of the field (in your opponents bunkers) to emphasize the angles and point out the blind shots. After you have walked the new field, play it. When the day is over, compare what you have learned by playing and watching other teams play the field, whether your team did a good job reading the field. With time, even this attribute will improve.