Your shoulders play a major role in the golf swing. If they are overly tight or weak, then they will get out of position in your golf swing causing major swing flaws and compensations. Strengthening your shoulders will not only help you prevent injury, but will also help you create a better golf swing.

Stand up against a wall with your arms bend at 90 degree angles.
Slowly raise your arms straight up, trying to keep your back to the wall, extending your arms as high as you can. Hold for a moment and drop arms down to start position.
Repeat the motion as you keep your spine stable.
Do 8 to 10 repetitions.
You can do this without a wall.