Coming “over the top” is easily the average player’s most common fault. From the top of the swing, the most common mistake I see is people initiating the downswing by spinning the shoulders and upper body towards the target. This forces your hands and arms away from your upper body and brings the club to the ball from the outside. Depending on your clubface, the result is either a slice or a pulled shot. This back arm under front arm drill will counteract the wrong move to help you prevent "coming over the top".

Start in your normal stance.
Hold your front out in front of you as shown and hold a club in your back hand.
Make a practice backswing with your back arm only.
Start your downswing and swing your back arm under your front arm as shown.
If your back arm swings over the front arm, then you have done the incorrect sequence. The back arm should fall under front arm.

This is an isometric exercise that will not only train the proper movement but will tone muscles in the hips, shoulders and chest.