Vanilla and BDSM

I suppose it depends on how kinky you are, and how vanilla your partner is. I would start with lowkey things (a blindfold, as Leil@ suggested, or a light wrist tie, or mild impact play like a couple spanks in doggy-style, etc). Let them know how much you like it. Most people like it when their partner is obviously enjoying something, and that can become a turn-on for them even if they aren't into the kink themselves.
If they are into dirty talk of any kind, sharing some explicit (but not TOO hard-core) fantasies of what you'd like them to do to you, and how aroused you are by it, might get the brain gears turning.

Discuss with them that this is a really fulfilling thing for you sexually and that you want them to be a part of making you feel good like that, but also listen to their position. In cases where the vanilla partner is just not very knowledgeable with kink yet, or isn't super kinky themselves but could be open to persuasion when it comes to their partner's pleasure, it's often easier to pull them in with a gradual introduction. If your vanilla partner has hard limits against certain kinks, though, then that needs to be respected and you should figure out a way to satisfy yourself without forcing them past their own boundaries. (Which is the same in any kinky relationship, of course.)

Slow and steady wins the race!