Most sports have become year-round — if not in terms of competition and play, then certainly in training. As you pointed out there are only two seasons: off-season (or preseason) and in-season. Each athlete should participate in a training program that includes a general overall body conditioning program as well as a sports-specific training program. This is the ideal way to prepare for competition while preventing injuries.
The first order of business on the prevention side is to make sure you have the right kind of shoulder and arm motion needed for throwing. Gentle stretching exercises are needed to keep the arm limber throughout the season. You won’t be surprised to know that strengthening the muscles of the entire upper extremity (arm) including muscles surrounding the scapula, shoulder joint, upper arm, elbow, forearm, wrist, and hand is the second order of the day.
What you probably don’t realize is how complex the muscle activity is when throwing a ball. Besides contracting and releasing, these muscles must also help the arm decelerate (slow down) at just the right moment. Each muscle has its own unique jobs that require different types of exercises to strengthen and train them.
On the prevention side of the equation, core training and lower body strengthening are keys to off-season training and in-season maintenance. Anything that happens in the lower body is going to affect the upper body and especially that important throwing arm. The entire body must be tuned, strong, flexible, stable, and hold up under strenuous conditions (a sign of endurance). There isn’t one exercise that addresses all of these functions. That’s why many athletes and teams rely on sports physical therapists, exercise physiologists, and athletic trainers to design the right program for them.
Other off-season activities should include rest for sure — usually right after the season. This is followed by a full-body conditioning program. Your goal is to build up your strength, power, and endurance. The idea is that with this type of approach, you will be able to compete during the season in top form while recovering quickly after the season. Stay active in other recreational and sports activities but take time off from the lacrosse and tennis.