Get yourself a rocker board...real good tool.
http://www.ballsnbands.com/theraband_rocker_board.html
Very cool, David - I've been using a wooden rocker board at my gym, will have check this out.
I think it is probably the same thing/concept as the rocker board you are using Hernan. I find it to be a great strenghtening tool though.
Try using the Bosu ball. It is a half ball that strengthens your core through balance. I started out doing small squats and moved to one leg balance. It really improved the muscles without stress on the hip joint.