Re: Developing a real serve at 18
The easiest way to fix a serve is to break it down into individual parts. For example: hit about 100 balls just work on your body rotation, toss the ball and twist your torso away from the court, and then twist back through while hitting using a simple motion. Next hit about 100 balls just bending your knees and exploding into contact (no body rotation). Then hit about 100 balls working on proper pronation (up and out as opposed to snapping down or brushing forward). Then another 100 balls just tossing in front and getting your weight behind the serve. Afterwards you have to slow start integrating them together, and I suggest doing it in the order I listed. It will take a very long time, probably a week or 2 hitting serves every day, but it is the easiest way.
100 balls for each might be a bit excessive, you could probably get away with 50.