Try strengthening up the intrinsic muscles of the rotator cuff. Sometimes they get weak because the bigger muscles try to do their job for them, which leads to overstressed muscles and injuries.
Here are a couple things to try:
1. Internal rotation. Tie a TheraBand to a chair about shoulder height. Hold it in the hand closest to the chair. Form a 90 degree angle with your arm, lining your elbow up right under your shoulder. KEEP YOUR ELBOW LINED UP WITH YOUR SHOULDER. Using your hand, pull your hand towards your stomach. Do this 5 - 10 times.
2. External rotation. Use the hand that's farthest from the chair to hold the TheraBand. Line your elbow up with the shoulder again. This time, pull your arm away from your body, KEEPING THE ELBOW LINED UP WITH YOUR SHOULDER. This is usually a bit harder. Do it 5 - 10 times.