Try to see your racquet strike the ball and keep looking at the point of contact for a split second afterward.
Most beginning servers (and many players who have been serving for years) meet the ball much too low. If you have trouble meeting the ball at full extension, practice tossing it to the right spot with or without a wind-up (whichever you do when you serve), and then freezing your racquet right where it meets the ball. This will help you realize where you're actually meeting the ball, and once you start meeting the ball at the right spot, it will help you remember that fully extended feeling.
