Get a piece of surgical tubing about a half inch or inch through the middle and about 3 feet or a meter long. Hold one end in your left hand and the other in your right. Now, keeping your upper right arm close to the body, swing your right arm, from the elbow down, outward like a hinge. This strengthens the shoulder muscles.

This will force the shoulder muscles to hurt, but it's a good hurt. Do this several times a day: queuing for the bus, in church during the sermon, during sex, while performing, etc. When it doesn't hurt, hold the tubing a bit closer to the other end.