On the back of your shoulders is the Supraspinatus, your rotator cuffs. This muscle risk injury if you practice racquetball or spend a lot of time in front of a computer. If you stretch your rotator cuffs often you can prevent or treat injuries. This is a simple stretch exercise you can do when laying in the couch or the bed.
Lay on your left side with your upper arm 90 degrees from your body and your forearm pointing up in a 90 degree angle. With your right hand, gently push your left wrist down while maintaining a 90 degree angle in your elbow. You will feel tension in your rotator cuff, the back of your shoulder. Hold the position for the desired amount of time and then switch to the other side.
One side is probably more tensed than the other and focus more on that side and strive for symmetry.
If you want you can place a pillow under your head for support.