A full shoulder turn allows you to coil your body and begin shifting your weight onto the back foot. Failing to turn properly can lead to weak or off-target shots. Improvement is possible with practice on the range -- or even at home without a golf ball. For the best results, ask a PGA or LPGA professional to review your swing.
Place the clubhead of your driver on the ground while positioning the shaft so the end, or butt of the club, points up, perpendicular to the ground. Extend your left arm straight and place the palm of your left hand on the butt of the club, if you are right-handed. Extend your right arm straight in front of you as if you are about to shake hands with a friend. From this position simply turn your right arm as far right as possible while allowing your shoulders to turn naturally. Hold your position for a few seconds. PGA Tour pro Stewart Cink uses this drill during his practice sessions.
Take your golf stance without holding a club. Your feet should be about shoulder-width apart with your arms hanging naturally. Place the palm of your right hand on your left shoulder, and place your left hand on your right shoulder, as if clutching your chest. Turn your left shoulder until it is roughly over your right foot.
Set up with your driver as if preparing to hit a shot. Take the club back slowly as you begin the back swing. Keep turning until your left shoulder is roughly where your right shoulder was at address. Keep your left arm extended as you bring the club up and around your body. Stop at the top of the back swing and hold your position. Look across your left shoulder to see how it has turned fully with your back to the target.