To help players add more topspin to their serves (and improve their follow-through), have them kneel down on the baseline (because our courts usually are about 120 degrees, I use towels for them to kneel on). Because players are suddenly about two or three feet tall, they will have to add more lift to their serves. This also encourages servers to keep their heads up and follow through across their bodies instead of straight down, because they will cause some serious damage to their racquets if their follow-through is incorrect!