Having problems making contact on the center of the clubface with your driver, and that's costing you distance and accuracy?
HOW YOU KNOW IT'S HAPPENING
When you hit it off the heel, it'll feel like you just hit a brick and the ball will shoot dead left. If you hit it off the toe, you'll feel a soft impact, like you whacked an old apple.
HOW TO FIX YOUR HEEL AND TOE HITS
Lay four dowels (or pool cues or even string) on the ground as shown. The spaces between the dowels should be slightly wider than your clubhead. Address an imaginary ball in the middle lane. If you tend to hit it off the toe, take practice swings making sure your clubhead travels through the far lane after impact. If you hit it off the heel (or tend to shank your irons), your downswing path should go through the near lane. After 10 practice swings, tee up a ball and swing away. Now that you're slotted correctly, you'll hit it square and the same distance every time.