In order to see the line of the putt and make a straight back and through stroke, it is essential that the eyes remain directly over the ball.  If your eyes are inside the line of the putt an inside-out motion may result, and conversely if the eyes are to far across the line of the putt a outside-in stroke may result. Either motion is destructive to consistent putting. The most consistent motion is a square-to-square motion where the putter travels on a straight line back and through the ball on towards the target.

One sure fire way to accomplish a square-to-square motion is to place a mirror underneath the ball.  As you assume your address position you now can easily get the eyes directly over the line of the putt.  Also the mirror can assist you on keeping your shoulders parallel to the ball-target line (another vital key in a consistent stroke).