Anal sex is a sensitive issue in so many senses of the word, and the approach -- both before the act and during -- has to be equally as delicate.
You could decide to go up to every single girl you see and ask her how she feel about butt stuff, and if you're persistent enough, you could probably produce positive results once or twice in a lifetime.
However, a more tactful approach is probably going to work better. Getting someone flowers or writing a heartfelt note is going to set you up for more positive results when you finally broach the topic -- as long as that note doesn't contain a slightly less heartfelt secret code.
Apologies in advance for the pet names used in the note; they may cause nausea and vomiting. I almost hope this is fake because I hate to think there are people in the world who call each other those names.