Use a NON-bouncing med ball! The object is to start from a static position and throw the ball as hard as possible. Before you throw brace your stomach and get your lats tight then launch the b-ball as hard as you can into the wall.
This is great for developing power in any type of overhead athlete.
Keep the med ball light, speed is the object. Using too much weight defeats the purpose and puts the shoulder at more risk for injury.
Start at 3x5 on week one and add a set or two each week.