The best place to start is asking: "Have you ever wanted to try __?" Start with something light -- tied hands, or a blindfold, or just tell her she has to hold still. Even "vanillas" would probably enjoy this a bit, and it can lead quite naturally to talks about doing more. If she's open to "new things," this shouldn't be a big deal. Worst case scenario, she says "No, I don't think I'd like that," and you move on.

Don't hint. That's not just for BDSM, that's for relationships generally. If you have to play games to get your message across, it's not going to work. I won't go on a rant on that topic, though, much as I'd like to.