This is a fun video that demonstrates how to add the right amount of under spin to you slice forehand or backhand. The key is you don't want to add too much cut on your ball, which is what most club players try to do. This exaggerated cut motion results in a ball that floats too much and has no pace.
What you want to do is be much more subtle with the amount of cut you put on the ball. I compare it to putting just a dash of salt in your soup. If you just add a dash or a hint of cut to your slice you will end up with a shot that more resembles what the pros like Roger Federer hit. A shot that has some drive to it and skids and stays low when it hits the other side of the court. Make sure to watch the whole video so you can see exactly what I am talking about and please be sure to let me know if this video helped!!!