FIFTEEN LOVE

Hold only one ball at a time.
It is inadvisable to hold two on a first serve for two reasons:

First, if the serve goes in, either you have to play the point carrying a ball in your hand or else you have to remember to throw it aside. Why do that?
Second, with another ball in your hand if you miss the first serve, you’re inclined to rush the second serve instead of taking your time as you should.