A good serve is heavily reliant on a consistent ball toss. Paul Annacone suggests tossing the ball between the 12:30 and 1:30 clock mark (for righty-handed players).
"We want to keep it as simple as possible," Annacone says.
It's impossible for the rest of your service motion parts to flow smoothly if your toss isn't right, and it's certainly easy enough to practice it.
Annacone was the coach of Roger Federer and Pete Sampras, one of the best server's in history. He's now working with Taylor Fritz.