Here are the ones I do:
You need one of those big balance balls and one of the half balls.
This:

and this:


Flip the half one over and hold onto the edges with your hands, put your feet up on the balance ball.

Do your push ups! <img src="/ubbthreads/images/graemlins/laugh.gif" alt="" /> <img src="/ubbthreads/images/graemlins/eek.gif" alt="" />