Focusing on the "shadow" underneath the ball is a surefire way to keep your head still through impact.
Cheryl Anderson
Many golfers move their heads too much when putting. They watch the ball leave the putter face on its journey to the hole rather than completing the stroke first and then watching the roll of the ball.
One of the best putting tips that I have ever learned to combat this tendency came from Fred Shoemaker, owner of Extraordinary Golf.
Peeking too soon can lead to your putter face remaining open to your target at impact. Rather than telling someone to keep their head still, I have found it more effective to have them focus on the spot under the ball after they reach impact during the stroke.
How does this work? You will notice a “shadow effect” on the ground after the ball leaves that space. Some describe it as a “halo-looking” shadow. The shadow lingers for a split second, which is enough to keep your head steady as you complete your stroke.