The most common cause for missed putts is movement. Golfers change their original posture in an attempt to see where the ball is going. This is suicide. At practice be aware of your posture as you line up your putt.  Make this a habit and retain your posture till after the putt is executed.  Infact, train yourself to hold your follow-through (however tempting it may be to check to see where the putt is headed) for a few seconds after your putt has been executed.
There are three elements that you have to constantly keep in mind while executing a putt.  Correct body position, putter bead path on target and putter face square to the target. All errors on the green are due to one or more of these aspects missing.