Here is a nice little thing, actually line up your putt were you think your going to hit in, then look at the hole and hit the putt with out looking at the ball. When we throw a dart, or throw a football, or basketball, we don't look at the ball we look at the target, thats because we can percieve depth with our eyes. That works because our eyes are offset from each other, meaning there's a gap, between them. This causes two images to be one, if there offset then you have depth perception, which everyone has except those who have only vision in one eye. Same principle in 3D movies
But what looking at the hole does is gets you to know visually how far you are from the hole, so when you glance in your preshot routine you can get the memory. Just keep putting looking at were you want the ball to end up, usually about 12-18 inches behind the hole is optimal, try to get your putts to end up there. Do that till you can get, lets say 5 in a row to end up near the distance. Then move and do it again.
Also during this process, DO NOT CHANGE YOUR PUTTER, OR YOUR PUTTING STROKE.
First thing first, get a consistant putting stroke, no way you can gauge distance if you can't hit the ball solid each time, your jsut then giving your head and muscles false information.