When I practice serves, my goal is usually to improve the efficiency of my motion. My go-to serve is my hard kicker. The effectiveness of this type of serve tends to correlate strongly with high high on the fence it hits.
The higher it hits on the fence, the heavier the serve. The heavier the serve, the higher the raccquethead speed. The higher the racquethead speed, the faster the ball dives downward into the box. The faster the ball dives downward into the box, the higher my serve percentage.
So I usually pick elements of my motion and focus on tweaking them to see how it effects the ball trajectory (as measured by ball bounce). I work on finding the optimum toss location. Optimum toss height? How much can I lean my hip out into the court? How much can I toss the ball to my left to get better back arch without losing racquethead speed? How wide should my stance be? How far into the court should I be landing?
I like to pick only one or two of these elements per session and try to always end the session with improvement in ball bounce from the beginning of the session.
Other times, I might work on optimizing the sideways kick of my twist serve. This also depends on maximizing racquethead speed, but it requires a different trajectory with more spin and less speed for optimum effectiveness.
As long as I am always working on a specific element, there is something to hold my interest.
BLX Blade 98. 13.69 oz., 12.55", 369 SW. 16x20 (outer mains skipped)
Pre-Stretched Ashaway Kevlar 16g/ZX Monogut Natural 16g, 66/46 lbs