When my kids started playing, their coach at that time thought they shouldn't compete until they could regularly "beat the ghost." Basically he would have them play games -- but with no one on the other side of the court! So as long as they did not double fault too much, they would win the game, or beat the ghost. We still have great memories of them out there, all alone, calling out the score.
But ultimately it did give them confidence about their serve once we started putting them into tournaments.