If you've had any sort of shoulder injury in the past, you should be **extremely** careful with stretches to increase shoulder flexibility. The one that comes to mind initially is where you hold your hand palm up, grab a rod between your thumb and index finger, then drop the rod clockwise is you are a righty and counterclockwise if you are a left until the rod is against the side of your upper arm. At this point, your forearm and the rod will be vertical. Grab the rod with your other hand at some point below the opposite arm's elbow and pull the rod across your body. This externally rotates the shoulder.
I've put myself out of commission by doing this exercise mildly. After a lifetime of throwing and racquet sports, it doesn't take much to hurt a shoulder that's been injured before.