Don't even try to talk her into it. Just go down on her and when she is hot and ready, just slip your tongue down there and go for it. Even if you have to slide your tongue up and down really fast, just get closer and closer to her butt. She will get used to it and it will let her experience how it feels. Once you have tongued her move on to massaging her anus with your finger while eating her. Don't try and stick it in, just massage her for a couple of times then finally slide your finger into her after she is used to it. After getting her used to that then you can go for the anal. You will at least have to get two fingers into her first though. (my experience)