Rest is first, but I'm sure you already tired that. If you can get your hands on a mini-TENS unit, that could also help with the muscles. I just got one from Propera off of Woot, and I love it. I use it a few times a week, and it definitely makes a difference and that's on my post-op shoulder.
Are you sure it's the muscle itself? Tendons and the rotator cuff are a very complicated mechanism...
Mt Snow, VT - May 5, 2012 - Completed!
...i remain, he who remains to be...