This stroke seems to irk my clients more intensely than any other error in the game. Ironically, it's the best 'bad shot' in golf since, unlike the others, it comes from a good swing which may often get down the course quite effectively.

Check that your left heel stays close to the ground during the backswing.

'Topping' can also be an aversion response to the greater evil of gouging into the ground or hitting 'fat'. So review "Bucket", "Sergeant Stripes" and "30° Run-On".