Here's another great drill idea from Cosmin Milhoca at WebTennisDrills.com. It is similar to the Keep It Deep serve drill, but adapted for a single player for either practicing alone or during a private lesson.
Player serves out a game against an imaginary opponent. First serve must be hard enough that it bounces 2 times or fewer before hitting back fence. Second serve may bounce 3 times or fewer. If either of these conditions is not met, serve is considered a fault. Double fault = point for 'opponent', natch.