When you take your grip, pull your left thumb (for right handed golfers) up a bit closer to the rest of your hand and fingers. The thumb wants to feel like it's not extended down the shaft at all. This is called the 'short thumb'. What this will do is at the top of your swing the pivot or hinge point will now be reduced and keep the club from dropping down too far on your backwswing. The short thumb makes for a shorter swing and gives you better control at the top.

Most folks have an over swing because they fold the arms too much, pick their arms up instead of turning their shoulders or sway their legs back too much but if you're pretty sure that these elements of your swing are pretty solid then the thumb trick may just do the job for you.