I'm part of a group of guys who play baseball almost every weekend. It's supposed to be just a fun activity but we're pretty competitive. I'm one of three pitchers on our team, so there's even a bit of competition among our own team members. I'm interested in finding out what I can do during the off-season and pre-season to keep my pitching arm in shape.
As you already know, a shoulder or elbow injury (the two most common types of injuries in overhead throwing) can put a pitcher out of the game. Sports orthopedic surgeons have investigated the effect of preseason shoulder strength on the risk of in-season throwing-related injuries.
After looking over what's already known about shoulder (throwing or pitching) injuries, they decided to see if weak shoulder muscles during preseason are linked with in-season problems. To test their ideas out, they measured the shoulder range-of-motion of 144 baseball pitchers. These were major and minor league baseball pitchers but their findings might still be what you are looking for.
All measurements were taken before the season began for five years in a row (2001-2005). Then they compared strength data with in-season injuries to see if there was a link between the two. They found that shoulder injuries were more likely to occur when muscles used to externally rotate the shoulder (cock the arm back to throw) were weak. An imbalance between internal and external rotator muscle strength (one group stronger or weaker than the other) was a red flag that weakness could lead to injury. This finding has been reported in many other studies as well.
Professional baseball pitchers also end up in surgery when the supraspinatus muscle is weak. The supraspinatus muscle is one of the four muscles of the rotator cuff. Its major function is to abduct the arm. Abduct means the arm moves away from the body.
Preseason strengthening of the rotator cuff and muscles of the elbow/forearm may be the answer. Strong muscles help reduce the force and load placed on the joints during overhead throwing activities. A strengthening and conditioning program is a good place to start. You may want to enlist the aid of a sports trainer, physical therapist, or other knowledgeable fitness instructor.