There is some more or less good advice on the net and some here on the forum. I was asking that question myself a while ago, bottom line I guess the best technique is to plant a cognitive virus, so she will think that was completely her idea at the end. That's the safest option more or less, this way you don't get any weird looks, refusal, etc... Of course I will get flamed for this idea by the community, but to hell - the result is what counts.
You actually need to bring up the subject often enough but in very small portions, during sex shop visits, during watching porn, over the dinner table you can throw in the health benefits, etc... The main point is to divide the whole subject into smaller chunks and continuously implant this "bite-size pieces" into the thinking process. After a while the person starts to bring up the subject and you can take it from there.

I don't know if my advice is helpful, I do a lot of NLP and for me it's quite easy, but there is a good book on the subject of conversational belief change, maybe you wanna have a look: Sleight of Mouth less

I'll be honest, my boyfriend was just like, "Hey, I like stuff up my butt. I hope that's cool because my previous girlfriend wasn't and it sucked." I have a try-anything-once kind of attitude, so I gave it a shot and now it's a routine part of our sex life.

Although at the same time, I can't promise that this approach would work for everyone. less