well, first of all, she has to get used to anything being there at all. this may take a week or two. have you tried your fingers? stick your finger in there while you are doing her. this works especially good if you time it when she is about to orgasm, because she is very relaxed. some positions work better than others. in my opinion, standing up, bent over, with a little reach around on the clit to get her there works very well. i think it also tricks the mind a little into believing it feels good back there. do this for a while, try 2 fingers, a very small dildo, work your way up until she stays relaxed enough to take your head. if the fingers and everything else get to feeling good, she will want more.
oh yeah, lots of lube too. i like "Wet" any flavor, but strawberry kiwi is especially good.
oh yeah, and once you do get in there, don't start thrusting like a crazy person, take it easy.