The great British champion Henry Cotton made this drill popular: Watch one of your thumbnails as you putt. He found that focusing on something closer to his eyes than the clubhead, which has a tendency to wobble distractingly, made the stroke easier to control. To create a stronger visual image, he actually painted one of his thumbnails with his wife’s nail varnish (it doesn’t matter which thumbnail).
This technique is also great for getting a handle on distance. With repetition, you begin to associate a thumbnail moving two inches back and two inches through with a certain distance and five inches back and through with another distance. Under pressure in a round, when your natural sense of feel for distance sometimes deserts you, this can be a lifesaver.
It’s also useful for sizing up the speed of the greens before a round at an unfamiliar course. If a two-inch thumb stroke rolls the ball more than what you’re accustomed to, you can make that adjustment.