From Bill Kroen, retired golf instructor and author of "Bill Kroen's Golf Tip A Day Calendar":

Many teachers try to instill in their students the fundamental of keeping the body and head still while putting. I have found that just saying "keep still" does not work. The key is to keep your eyes still during the stroke, then the body and head will naturally remain quiet. To accomplish this, I tell my students to look at the grass that was under the ball after the stroke has been made. In this way, you make a positive move to keep the eyes over the ball until it is already moving toward the hole.