I have a safe word. But this is what works best for us and I think it is a Godsend. Green...Yellow...Red.
If things are going really well I will say green from time to time to let him know all is fine. If, for instance, he was slapping my face a bit to hard for comfort...it is yellow. That means all is still OK but please proceed with caution Sir. Red is the same as my safe word meaning STOP! Nuff said. "Peace" ~blizz~