Here's some of the exercises we send home with our patients when they come in...
We'll have a patient lay on their stomach on a table, with their arm off the table, dangling towards the floor. Raise your arm up, (toward your feet, not outward) as far as you can, and hold it there for a few seconds, then lower again. They have a name for this one, but I cant remember it. I just call it shoulder hyperextension.
Here's another one. Bend over at the waist and let your arms hang to the floor. Then, retract your shoulder as far as you can, until your fist is even with your chest. The action you're looking for is similar to drawing back a bow, or even better, about to blast someone in the face with your fist. This one may be more of a shoulder muscles exercise...I can really feel my posterior rotator cuff muscles at work when I just did it. But it couldn't hurt to try it on that AC joint.
Also, make sure to stretch out really well before you start shooting. My pops has one of those Bowflex gadgets that he mounts before he shoots. That man has some serious shoulder issues! He's a direct example of what happens when you shoot a 65lb longbow for years, then stop for a year, then pick it up and try to pull it again. He's lucky to draw 48lbs now.
If I studied my notes well, that joint is a pretty common problem child for people starting at about the age of 50. I just turned 30, but mine is already giving me problems due to my brawl with a '96 Silverado.
If your interested, I can pick up papers from work that give more of a detailed exercise plan and either mail them to you or copy them down and email them your way.
Good luck man!
Cam