As per QE, the ladder drill is excellent.

One thing I'd consider is the stroke itself. Part of the reason your distance control is so erratic is probably because you are not allowing your instincts to control the club. You are either consciously or unconsciously over-accelerating (sort of "popping" or "jabbing" at the ball) or decelerating (pulling up or back because you don't trust your instinct) when you putt.

The thing is that putting is just rolling the ball. Your brain knows how far back to take the club, and you just have to get out of its way. This method works especially well if you don't change the acceleration of the club with your body, arms, or hands. Just let gravity provide the necessary acceleration. Just let your subconscious self take over -- it really does know what to do. With all the golf you've ever played, your brain has built a very large database of conditions. Just let it take over.

More on these ideas can be found in the excellent book Optimal Putting by Geoff Mangum or Geoff's Putting Zone webpage.

edited for spelling