The keys to great putting are really pretty simple. You must hit the ball squarely with the putterface, keep the putterhead moving level to the ground and square to your target line, and swing the putter at the proper speed to roll the ball the desired distance. To accomplish this more consistently, I recommend a grip that's a bit revolutionary, but extremely effective.
Position each hand so that your palms, if opened, would face upward and inward at 45-degree angles. In this position, each hand prevents the other from unduly opening or closing the clubface during the stroke. Grip pressure should be equal in both hands and constant throughout your stroke.
Now, bend your elbows until they're at 45-degree angles to the shaft. This will enable you to move the putter back and forth on a steady, accurate plane and direction, allowing you to put the intended roll on the ball. Play the golf ball off of your forward foot, with your eyes positioned directly over the target line.
I advocate keeping your wrists very firm throughout the stroke. A good way to help you do this is to re-grip your putter with a grip designed with a flattened top, and place it on the putter's shaft so that the flat side points toward the target. You'll find this to be a great help in preventing your wrists from breaking down during the stroke. Also, make certain that the length of the putter allows the end of the putter's handle to fit into the palm of your left hand (for right-handers), with the left thumb pressed firmly on the flat part of the grip.
You'll find this grip technique to be helpful when chipping as well.
Former touring and teaching professional Al Wagner founded Diamondhead Golf Club Manufacturing Company.