A massage therapist showed me a pretty good stretch to do to help with this (as it was my shoulders that were killing me when I went to'em for help).
Find a corner of a wall, and go stand by it. Hold out your arm, in the stereotypical cop-style "stop" fashion - i.e. shoulder-to-elbow is perpendicular to the floor, and your forearm is held straight up with hand pointing toward the ceiling.
Lay your forearm/elbow up against the wall that goes off to your side (not the wall that goes straight away from you), near the corner. Then put some resistance on your arm - lean in a little, and push against the wall with your arm, so that you're stretching out the muscles in your chest some.
Then simply turn 90 degrees, and do the same with the other arm on the other side of the corner of the wall.