The backhand slice

Grip: Continental
Hit outwards with a high to low knife-like motion.
45 degree angle of the racket face.
Step out with the dominant foot and lean into the shot.
Feel your spare hand extend out behind you for better balance as it counters the forward swing.

Hitting into the path of the ball leads to a clean contact than hitting across the path. Hitting across however, also adds some side spin.

A Slice will cause the ball to float in the air, making it easy to hit it deep.

If you want a sharper downward trajectory, make your string-bed tilt upwards more.
For more power, start the swing the backhand farther back

Ref:
BBC Tennis
Tennis.About.com