A great way to help you hit your putts in the sweet spot of your putter is to use 2 adhesive bandages on the face as a training aid.  Place the bandages on the face of your putter so that the soft pads are laying down the front of the face.  Leave about a one-inch space near the center of your putter.  In practice, stroke putts of about six feet in length.  If you strike the ball in the center, the putt will roll normally to the hole.  If, however, you hit the bandage pads, your ball will roll weakly.  Keep practicing until you can hit the center of your putter face almost every time.