Our athletic trainer at Lexington Christian Academy, Andrew Carlson, takes pre-season measurements on all his baseball pitchers - everything from shoulder motion and strength, to hip strength and evaluation of scapular mobility. He has done this each of the last 3 years, and along with a preventative arm care program, he hopes to prevent injuries through the season. There are a few issues that are frequently popping up, but this year he has noticed some tightness around the upper body. Manual stretching with your athletic trainer or therapist is important, especially as internal rotation and flexion tends to tighten through the season, but self-stretching and self-mobilization of the upper body is very important. Here is a good series of active upper extremity stretching:
Here are is another great video from Eric Cressey discussing the importance of shoulder flexion in throwers - a must watch for pitchers and therapists/athletic trainers!