Don't bend over.
Seriously, though, one my cats, Mochie
used to do that and she did right from the 1st day I got her. Fortunately she was a pretty light cat. I used to just try to not present opportunities for her to jump on my back meaning I'd brush my teeth standing up straight, etc. If she saw me hunched over or bent over she'd ALWAYS jump on me. I never found a way to stop it.