How do I serve a fast ball in Wii Sports?

To create a fast served ball (the one that's on fire), you have to hit the ball when it's at its peak. I don't have a specific strategy to do so, but that's how you do it.

It has a lot to do with timing. I like to use a flick of the wrist downward when the ball is at its peak. Takes some practice but once you've got it down you will have no problem doing it every time.