When working with the more advanced students, I teach the "Too" Serve.

1. Start the racket motion "too" slow rather than "too" fast. Starting the racket fast on the "take-back" will not help the ball go faster.
2. Throw the toss "too" high rather than "too" low. Throwing the toss "too" high allows you more time to hit it on the fall. Almost all overheads are hit on the fall.
3. Leave you toss arm up "too" long after you release the toss. The longer you leave the toss hand up, the higher you will want to hit the toss, the less you will bend at the waist and it will force you to keep your head up.
4. Come off the ground "too" much rather than not at all. As you develop power, but you are unable to control it, coming off the ground will help bring the ball down.
5. Snap your wrist "too" much rather than not at all. Think about your wrist being the fastest moving part of your body when you serve. This will protect the rotary cuff in your shoulder and allow maximum power.

Good luck on the court!