Thanks everyone for the tips so far.
Originally Posted by rkelley
an important thing to remember is that your "flat" serve should not really be flat - i.e. no spin. It should have some topspin on it to help pull it down into the court. Sampras, Roddick, Fed, etc. all have lots of topspin on their "flat" 120-140 mph serves.
How does one impart topspin with a flat serve motion. When I pronate, I hit the ball squarely(or at least that's what I think I do). I don't intentionally brush up.