You may have been told that you need to stay behind the ball at impact.  To do that you need to initiate the downswing with the lower body as the upper body stabilizes, but if the shoulders are tight or weak they will push the upper body towards the target first resulting in an over the top move or early release. Many swing faults can be corrected by incorporating some flexibility exercises into your daily routine.

Floor Shoulder Stretch

1. Lie on your back with your knees bent and feet flat on the floor and your lower back flat.
2. Extend your arms over your head, keeping your shoulders away from your ears.
3. Exhale as you pump your arms down and then inhale as your lift your arms up.
4. Repeat for 8 to 10 repetitions.