John Jacobs was once asked what, in retrospect, he would have done to improve his game. He said that he would stop trying to swing the putter up & down the line & let it swing on a shallow arc. A mate of mine, a good golfer, advised keeping the putter low to the ground. However, if you look at the pros on TV when they show a close up down the line shot, you can see the putter coming quite high off the green, in a shallow arc. The putter comes naturally inside when swinging back & I think it's wrong to fight that.