There are many parallels from sport to sport. Borrowing from some other sports here are a couple of tips you can use to improve your overhead:

Pretend that you are a football quarterback in the pocket. When moving back for the overhead make sure you turn your body sideways with your hands in a throwing motion. Keeping your balance and having your hands ready are both key to a good overhead.

Think that you are catching a fly ball with your non-dominant hand.  Making sure your left hand (if you are right-handed) is under the ball assures a better point of contact.  Make sure you are balanced and not hitting off your back foot.

Spike it like volleyball. Wrist snap is essential to a good overhead. Make sure that you make contact in front of your front shoulder, get your weight towards the court and snap! Make it a goal to have your overhead bounce just once before going over the fence.

This should be an aggressive shot that helps put you in an offensive position to end the point.  The overhead is a great weapon to have in your arsenal.  So the next time you hit the courts, remember to practice your overhead.