Good putters stroke putts using mostly their upper body. Bad putters? The ones I know tend to lack stability in their lower body.
Here's a great drill to help you understand what I'm talking about: Make strokes with the sledgehammer in your garage. You'll find out right away if you're moving your lower body too much, because the momentum of the head will pull your body off line.
Instead of letting the sledgehammer or your putter swing you off line, establish a strong base of support and maintain it. You'll quickly hammer home the importance of a sturdy lower body. You've got it right when you sense the hammer swinging like a pendulum, despite its mass. The smoother your stroke, the better your results will be.