“Hey ______, I need to tell you this: I really, *really* like being on the sub side of rough sex. I would absolutely love it if you would shove me up against a wall, wrap your hand around my throat and fuck me hard. My safe word is ‘platypus’, and until you hear that word I am asking you to do your level best to fuck me so hard I can’t walk straight for the rest of the day.”

Honesty will do the trick just about every time.