Fat and thin shots happen because the shaft passes the left arm before impact. The left wrist bends and breaks down, and the radius of the swing changes. The club hits the ground behind the ball, or you try to save the swing and hit it thin.
To keep your radius consistent, try this drill in a fairway bunker. With a marker, draw a line straight down your wrist, starting from the big knuckle on your left index finger. Then draw a line in the sand two inches behind your left heel. If you keep your radius the same length through impact--keeping that line on your wrist straight--your swing should bottom out in the same place every time, just in front of the line in the sand (right). Then simply play the ball where the line is, and you'll make flush contact.
TOM NESS is ranked 47th on Golf Digest's list of the 50 Greatest Teachers and is based at the Alpharetta (Ga.) Athletic Club.