One of my biggest worries is suffering some sort of injury that prevents me from shooting for several months. Apart from shooting with good form of course, whats the best way to avoid shoulder problems from shooting lots? I saw on a DVD an archer doing some exercise to strengthen the rotator cuff. It looked like she was lying on a mat, with a tennis ball under her shoulder, arm bent at the elbow and rotating the upper arm. I'm not sure how this was supposed to work exactly.

Also are there any good exercises that would help with flexibility in the shoulders - particularly the drawing shoulder - or can too much flexibility be counter productive?