One of the worst things you can do to your golf game is to tighten up or get too tense when teeing off, or when you're on the fairway or green. Instructors have a cure for this; it's a drill called “the hip blocker.”
First fix your knees as if ready to swing at the ball. This will force the body to turn more correctly and increase flexibility. If you're right-handed, while swinging, turn your shoulders back until the left shoulder is under your chin. You should be able to feel a greater stretch along your left side. Do the same thing on your follow-through, but this time with your right shoulder under your chin. With this exercise, in time you will increase your ability to fully coil without using your hips. You'll achieve a new flexibility and at the same time increase the power in your swing.