Two problems I see with slicers. First, they roll the clubface open on the backswing and don't close it on the downswing, which leaves the face open at impact. Often the reason is a weak grip, with the hands turned too far to the left at address. To fix this, set your hands more to the right--in effect, pre-set them in a more rotated position. As a result, the clubface will open less going back, making it easier to square up at the ball.
Another problem is the instinct to hold the clubface square through impact and swing straight down the line (see inset photo). This might make sense to you, but the clubhead has to swing on an arc, with the face rotating as it goes. So when you think you're holding the face square, you're actually forcing it open. The clubhead should swing from the inside, out to the ball, then back to the inside, with the toe passing the heel. Best advice: Take a light grip, and feel like you maintain that pressure. Then just let your arms roll over naturally (see photo).
Todd Anderson, a Golf Digest Teaching Professional, is director of instruction at Sea Island G.C., St. Simons Island, Ga.