Pitching is obviously a crucial part of the game -- without it you just can't win. And the game isn't nearly as fun for anyone (teammates, opponents, umpires, fans) if pitchers just can't throw strikes.
Remember, the point of these drills is to develop good mechanics. Without good mechanics you will have trouble throwing strikes consistently and your arm will tire (and damage) more quickly.
These drills might be simple but they are done by professional pitchers to reinforce their pitching mechanics.
If you don't have access to an indoor baseball facility you can do these with a tennis ball or soft-core rookie baseball in your basement.
Pay attention to the technique. Use your iPod / smartphone to record a video of you doing it so you can make sure you have the right technique. If you're not sure, email the video to your coach and they can comment.
Shoulder Strengthening Drill
This is a good drill to strengthen your shoulder and help protect it once the season starts.
NOTE: Younger kids may not be able to make it through the whole drill and since they can't build muscle there is no sense in trying to "man-up" through it. Have them go through it as far as they can with minimal discomfort.
One Knee Basic Arm Action Drill
Really basic but ultra-important. With the Bulls I see a lot of kids who have loads of potential but aren't close to realizing it because their basic mechanics are not good. You can do this by yourself in your basement with a tennis ball if you have no one else to help.
Make sure your hand comes out of your glove going down and just before the ball comes up and your shoulder rotates that your hand trails your elbow. In other words your elbow should be further away from your body than your hand.
One Knee Basic Arm Action Drill
One Knee Tee Basic Arm Action Drill
If your coach has ever told you that "you drop your elbow" or to "drive the ball down" this is drill you need to work on.
If you don't have a tee put some pillows on top of a box. Just have something soft so if you do it the obstacle with your elbow you don't hurt your elbow.
One Knee Tee Drill