A very good drill for practicing forehand groundstroke variety and spin control.
The ball machine will feed balls deep to the baseline, forehand side.
I will begin the drill from behind the baseline center and I will alternate hitting one ball with topspin and one with slice, aiming a few cross-court, then down-the-line, ending with alternating 2 cross-court and 2 down-the-line.
Technical aspects that should be noticed:

- keep the feet moving as you wait for the ball; I prefer open stance for my topspin shots, and square or semi-open for the slice groundstrokes;
- for the topspin forehands keep the wrist relaxed and brush up on the ball; the grip is semi-western;
- for the slice groundstrokes, I use a continental grip, and (very important) prepare with the racquet higher than the eventual point of contact so that the swing should be done from high to low and forward to impart underspin on the ball; the result is a low bouncing ball.

Secrets: 
- Topspin Forehand Groundstroke: Cradle the racquet neck (when preparing for the ball) with the non-dominant hand to limit the backswing and load the upper body which will result in more power.
- Slice Forehand Groundstroke: Just like the previous shot, I like to hold on to the racquet neck with the non-dominant hand until ready to swing down and forward - this helps with more racquet control, short backswing, clean contact with the ball.