A tennis ball machine is very useful for practicing as it is a good substitute for someone to hit back balls at you from the other side of the court. Prices vary depending on whether it is a normal machine or a tournament style one. The latter will, obviously be more expensive as it will offer more features. Some may be able to hold a larger number of balls, shoot them at different speeds and have more playing patterns. A few even come with a remote control. A tennis ball machine will help you to be on top of your game! Go here to find a tennis ball machine.