As a coach, good communication with players/parents about pitches thrown on other teams is key.
I'm a big proponent of the old-fashioned rubber hose exercises and stretches. Cheap and easy to transport means players will actually do them (sometimes). I set them up at practice so kids go through the progression everyday.
(Looking at your initial note, I'm not sure if you are a parent or a coach, but, if the former, the number one thing you can do is make sure your player is not getting overused, especially if they play on more than one team. I set up firm pitch counts for my sons on all their teams.)