Many players are familiar with the volley drill popularized by the Bryan Brothers. In this drill, one player start in opposite corners of the service/side line and work their way side-to-side hitting back and forth volleys. This is a good drill for learning to control a moving object with the volleys while the player is also on the move. The only caveat is that you're supposed to move FORWARDS at the net so performing this drill exclusively might not be the best thing for your volley game. However, in the same vein as the "Z" volley drill, one could practice (along with a partner) the "Z" Forehand Drill. This is where the players position themselves at opposite corners at the baseline and work their way side--to-side by trading forehand. In the diagram above, A feeds X; X hits to B; B hits to Y; Y hits to C; C hits to Z; Z hits back to C; C hits to Y; Y hits to B; B hits to X; X hits to A. This is the basic concept of the game. In practice, there will be multiple targets but the point is the players push each other around the court with their forehands and learn how to set up for the ball and exploit angles. To spice things up, try playing the point out after performing the drill for one minute.