In the golf swing, you need to take your shoulders through their full range of motion to create power. Any restrictions in your shoulders may cause a compromise the backswing and loss of power on the downswing.

Use a golf club, hold it across your thighs with arms extended.
Stand with feet shoulder width apart.
Raise the club above your head, extending your arms straight.
Slowly bend your arms at the elbows and drop the club behind your head so that the club rests on your shoulders.
Then raise you arms up over your head again.
Slowly raise the club up and down, taking your shoulders through their full range of motion.
Repeat for 8 to 10 repetitions.

This exercise will stretch your shoulders and upper back. This is a good exercise to perform before play or practice.