Find a really good anal video...one where the girls are really into it and enjoying themselves...look for something by Seymoure Butts...his chicks are for real and they always have a good time. If your girl is still nervous, find some posts on here for her to read. Also, start out small...maybe a very small (not much wider than your finger) vibe for you to use on her ass along with another one in her pussy...anal vibes in dark colors make a girl feel more comfortable for obvious reasons. If she doesn't want to use it with you, just leave it with her and tell her to think about trying it out when she's alone. After she finds out how good it feels she'll be more excited about letting someting as big as your cock in there, just take it slow, and remember, you can never use too much lube for anal!
Just some ideas...good luck!