Golfers who hit a lot of thin shots tend to swing the club too steeply into the ball. That's because they slide past the ball on the downswing and have to force the club down to make contact. When they slide too far, they catch only the top half of the ball, hitting it thin.
The problem actually starts on the backswing. Thin hitters usually lift the club abruptly, without much shoulder turn (NO), which leads to the forward slide. If you don't wind up going back, you have nothing to unwind on the downswing. So you slide.
Here's how to stop that cycle. Swing your arms back wide, and make a full shoulder turn. Try to get your back to the target (YES). Don't be too eager to hit the ball: Turn to the top, and then turn through. If you make a good move going back, you'll be in position to make a more level strike and stop hitting it thin.
Tee up some balls as you would for a driver, and practice hitting them with your 5-iron. If you move in front of the ball on the downswing, like thin hitters do, you'll swing down too steeply and slide the club under the ball, making contact high on the face.
To hit a ball solidly off a high tee with an iron, your swing has to be pretty level at the bottom. Make a full backswing and feel as if you're staying over the ball through impact.
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, Henderson, Nev.