When putting, keep your head down until you have completed the stroke. Make your backswing and follow through before looking up. Once you have completed your follow through, it's perfectly fine to watch the ball.
Hit Up On Putts
Many amateur golfers hit down on their putts, causing the ball to skip. Hitting up on your putts allows the ball to roll end over end. Put the ball forward in your stance to make it easy to hit up on your putts.
Watch a professional putt, and you will notice he takes the putter back slow and accelerates through the putt. Many amateurs miss short putts because they decelerate into impact. Accelerate through your putts and you will become more consistent.