Swing the putterhead straight down your line (above, left). Why? Because the first fundamental of good putting is being able to start the ball on the line you've picked, and swinging down that line is the simplest way to do it. The putterhead should move a little to the inside going back, then release down the line. By "release" I mean let the face rotate to the left as the putter swings straight. You'll give more putts a chance to go in.
Don't try to swing the putter on an arc so it goes back to the inside in the follow-through (above, right). If the putterhead goes left and the face rotates left—as it naturally does—you'll tend to pull the ball. I see golfers who get into the habit of holding the face open to avoid this miss. But that's adding another variable. Great putters have the fewest variables. Let the face rotate, and swing where you want the ball to go.
It gives you instant feedback. If you loop your stroke to the outside coming into the ball, you'll hit the outside tee. If you swing too much from the inside, you'll hit the inside tee. Great drill.
BUTCH HARMON, a Golf Digest Teaching Professional, runs the Butch Harmon School of Golf, at Rio Secco Golf Club, Henderson, Nev.**