My dogs spend their summers on the beach — three months of daily walks (weather permitting) chasing a tennis ball.  By the end of the summer,  my shoulder used to be totally burned out, until I discovered the Chuckit! An ingenious piece of plastic, the business end of it grabs on to a tennis ball, and holds it until the centrifugal force from a flick of your wrist sends the ball flying.  Not only does it save your shoulder from burn-out, it enables you to pick up the ball without stooping down, or handling slobbery balls.  Around $10 at your local
Petco,
Petco.com
or Amazon.com.