This is probably one of the most common putting errors, and it even happens on the Tour.
If you lift your head to watch the ball on its way to the hole you will most likely miss the putt. The reason for this is that lifting the head changes your posture and the pivot point of the stroke. Your alignment and swing plane are compromised in mid-stroke changing the putter head path and the putter face presentation to the ball. There is nothing good about lifting your head to see where the ball is going especially if this is done – which is often the case — during the stroke.
Look at a spot (blade of grass) immediately behind the ball before, during and after the stroke has been made.
A great training tool to help keep your putting stroke in plane and on track is The Frog Stick, one of my most recent innovations. Use it indoors or outdoors to build confidence and groove a winning stroke.