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.