Low Ball Toss

Here is a great groundstroke drill.  It will help correct many different problems: footwork, racquet prep, height and speed, and it helps a player increase depth. However, prepare to get pegged when your player tires.
You start with your player standing just behind the baseline at the center hashmark. You kneel about 6 feet in front, off to the side with a 60-ball hopper. Toss low balls to the player: your toss should be less than two feet high, and dead balls work best. Player shuffles, hits the ball on the rise and shuffles back to the hashmark. Repeat until the hopper is empty, varying placement from standard feed, to wide, to at the body. As the player tires, it's important to remind them to get thier racquet back early, bend thier knees, and to keep thier back straight.
Surprisingly, players love this drill - it's exhausting for them, especially on the backhand side, which requires more steps. If a player can hit all 60 balls in play, watch out, you have a soon to be superstar on your hands.