Cassy used to like standing on my back when I'm stooped over and gave it up for a while. He's back doing it again. If I have the time, I tickle his tummy or his paws till he jumps off. Otherwise I straighten up VERY slowly.

He particularly likes doing this when I'm cleaning the litter pans. Except when he's impatiently waiting for me to finish scooping so he gets 'dibs' on the clean pan.