Repitition is definitely something that will help...getting on, getting off, getting on, riding off a few steps, stopping, getting off, etc...teaching the horse not to expect when either is going to happen; he may simply be anticipating your mounting, and thus taking off on his own, without really waiting for your cue. So do it over and over again, in various places...
If he just moves as you are swinging over, stop him, back him up, or maybe do some leg yields, ask for a stand, and get off, and start again. Everything is about teaching him to stop anticipating moving off until you give the cue.
If he won't let you get up at all, before taking off, I second making him move; backwards, fowards, sideways, etc...make him realize that standing still is the best choice; horses are naturally lazy, so he will figure it out quickly.
"The ideal horseman has the courage of a lion, the patience of a saint, and the hands of a woman..."