Be patient, start with a little tongue here and there. If you get a favorable reaction (and you will) then make sure you are very communicative with her, ask her if she likes it, she will say yes. This will make her more psychologically comfortable about her ass in addition to the dirty talk turn on. Over time slide a little bit of a finger up there while you lick her pussy, just a little bit don't make it a big deal. Then, a week or two later try it again, maybe work the finger up there just a tad bit more, but stay cool about it. Over time, but not every time, give her just a little ass treatment. Eventually she start to wonder when it is the next you're gonna play with her ass.
At some point, and you'll know when the time is right, you'll say something like "well, since that feels so good up there then just how good it might feel with my dick in there..." I think you get the point. Try my system, I believe you will like the results. If she doesn't go for it, no big deal- just cum on her face.
Oh, yeah, Alcohol and a copious amount of lube will really help.