If you address the ball the same way every time and practice good mechanics until they become second nature, you'll soon find that no four-footer will get you shaking. Here's how Johnson Wagner improved his make percentage.
Try this drill: Find a left-to-righter and stick a tee several feet down your line. Align your body just left of the tee and forget the hole--focus on your contact with the ball. Make sure the putter releases so the face is square through impact.
To achieve perfect impact on short-game shots, it's all about lag. Should the clubhead lag behind your hands into impact, or should your hands lag behind the clubhead? It depends on whether you need a powerful shot or a soft touch.
Sometimes simply willing oneself to stay still doesn't get the job done. When that's happened to me, I've often solved the problem by making a conscious effort to set my weight on the insides of my feet or on the balls of my feet.