place a block of wood or some object level with the back foot. After an hour or so you will either continually hit the block of wood or you will achieve the result you are after. OR find a three foot putt on the flattest part of the practice green or the carpet at home and take no backswing at all, simply push the putter at the hole. It is illegal to do this in competition however is an ideal drill to make you accelerate through the putt