If your backswing is short or your turn in restricted, then you need to practicing making a better turn. During the swing the power is generated from the torque you create between the upper and lower body. This exercise is a great exercise to help strengthen the core and shoulder stabilizers simultaneously.

Stand in a shoulder width stance holding a golf club parallel to the ground with your arms extended away from your chest.
Stabilize your lower body as you rotate your upper body independently from your lower body.
Turn your torso as far as you can in one direction, then slowly repeat toward the opposite directions.
Repeat rotating from side to side for 8 to 10 repetitions on both sides.
This is a great exercise to perform before you play or practice. You can use rubber tubing to add more resistance and challenge the strength in your shoulders and chest.