Most fat shots are easy to diagnose. If you don't shift to your front side on the downswing, the club runs into the ground before it gets to the ball—and you chunk it.
Golfers make this mistake because they try to help the ball into the air. As they start down, the weight goes to the back foot, and the head and spine tilt back. They unhinge the wrists early, which sends the club into the ground.
To stop doing this, you have to trust that the club's loft will get the ball up. Shift to your front side as your first move down. Here's a good swing thought: Swing to a finish. Focus on finishing with your weight left and your body over your left leg, and you'll start hitting it pure.
If you tend to catch shots heavy, hit balls off a downslope. Because the ground behind the ball is higher, you'll learn to shift forward—or else really hit it fat.
Ranked No. 1 by his peers among Golf Digest's 50 Greatest Teachers, Harmon runs the Butch Harmon School of Golf at Rio Secco Golf Club in Henderson, Nev.