affix robot to the floor

Hi, I have a robot torso that I want to affix to the floor. You can see a picture at http://www.cs.utexas.edu/users/jmugan/DevelopmentalAgent/

I tried making it really heavy and increasing the friction but it still moves a little bit. I'm sure there is a simple method that I am overlooking. Anyone have any ideas for what that might be?

Thanks,
Jonathan