That old admonition, "keep your eye on the ball," is not nearly as meaningful as it has often been conceded to be. The key point is not merely to keep your eye on the ball but to keep your head firmly anchored throughout your swing. Why is this so important? Well, the head serves as the pivotal center of the swing. As long as the head remains still and steady—and assuming your swing is built on a fundamentally sound action—you cannot fail to hit fine golf shots. However, if you move your head, you throw the center of your pivotal action off-center, and this forces you to set up a series of compensations in your swing. You can get away with a swing made up of compensations now and then, but you are really defeating yourself in the long run. Consistently good golf can only be produced by sound, correct action.
One excellent method for testing whether or not you are keeping your head steady is to line it up on some nearby object (like a tree), then stop when you are about halfway into your backswing and check whether or not your head has maintained that alignment. When the sun is out, the check is made much simpler. Then you line up your "shadow head" on some object like a tee marker and check whether or not it strays off the mark as you move into your backswing motion.
from HARRY COOPER, Metropolis Country Club, White Plains, N.Y.