Go slow, be careful. Use some fingers beforehand to make loosen her up (both mentally and physically). Also, the more turned on she is, the looser she will be... and if she's nervous, it will be tighter. I've never had a problem with it causing too much pain with virgins... just take it easy and she'll be fine.
everything what moravian said, also have some KY to hand! i've heard this a few times too: just as your penetrating her, bite her ear lobe to distract her from the slight pain she'll feel down there
Find a Viking helmet with the horns and sing volga boatmen really loud, that'll distract her plenty till you're in. After that you mumble about how you're going to take her to maui and chew on her ear. It always works for me.
My method would be to eat her pussy for 1 hour and she will be soooo wet!!! you can slip and slide right in and she will be relaxed. get her used to the idea of taking it in the ass early and you wont have to talk her into it later... and then what you do is blast a huge glob of steamy Man-Chowder across her face!!! woo-yea!
