Ask. Don't plead or beg = not sexy. Make sex fun and pay attention to what she digs so you can make it super exciting for her. Be an awesome lover. Then, she'll be more likely to do it out of appreciation, which is really the best way to get something.
Also, don't just ram it in. It hurts SERIOUSLY. I think guys underestimate how badly it can hurt if he goes about it just like he's banging away. You can't approach anal the same way you approach vaginal sex. Lube is good too. Be gentle. If you just try to stick it in and hope she "doesn't notice" or "goes along with it" she'll end up kinda resenting you and that will make your overall sex life suffer. So do it the right way. Get her all hornied up for a while, talk about how she feels about anal, and then explore it in a sensitive way. Experiment with all kinds of stuff -- let her take the lead for a while with things she wants to try out too. It'll actually be more fun for you that way, as well as make your overall sex life improve.