Problem: When hitting overheads and serves, many players have a difficult time getting in the correct position. Typically, they let the ball get too low and you will see them drop their lead arm and bend over at the waist.
Fix - Overhead: On your overhead, fold your lead arm at the elbow and point the tip of your elbow at the ball. By doing this, it will force you to see and hit the ball higher. It will also remind you to get under the ball so you can hit with your arm extended and allow your wrist snap to work.
Fix - Serve: To fix the same problem on the serve, I find that reminding the student to get under the toss can really improve their serve. Try tossing the ball in a position where you feel your abdominal muscles slightly contract. It is not necessary to throw the ball so far back that you hurt your back. If you imagine that your belly button can look up and see the bottom of the ball, not the back, it will help, doing this will help you to "hit up" on the ball. This will force your ball to have a higher arc over the net and have a very unpredictable bounce for your opponent.
Problem: Inconsistent Overhead. Many players will lose confidence on their overhead, when in many cases the ball they are hitting is not actually an overhead. Many times in a match you will receive a high ball that has a shallow arc. Because of the shallow arc it will not give you time to drop your racket head and snap your wrist.
Fix:I teach my students to see this ball as a high, stiff-arm volley. This shot is very consistent and works well when under pressure and under poor lighting conditions, and it allows you to move back to the net to control the offense. It does not have to win the point directly. This alone will help your confidence.
Good luck on the courts!