Increase your shoulder turn. (Look at Tiger)
The main reason Tiger Woods creates a tremendous amount of clubhead speed in his swing is because he turns his left shoulder over his right knee at the top of the swing. To do this, one must be flexible in the shoulders and back.
Stretching improves the flexibility of the muscles and joints necessary for a smooth, rhythmic golf swing. A complete fitness program should include stretches for your legs, back, shoulders and arms. One of my favorite stretches is a simple way to help increase your shoulder turn – and help your everyday posture.
Standing straight, hold a golf club behind your back with your left arm above your head and the left palm facing toward you. Put your right arm behind your back with the right palm facing out. Hold this position for 30 seconds. You should feel a stretch in your shoulders, back and chest. Switch arm positions and repeat.
Tiger can actually do this stretch without a golf club; he is flexible enough that his fingertips touch behind his back. Do this exercise day daily.