What ever you do make sure she's prepared for it and relaxed. You gotta be pateint with her both when asking and doing. If she isn't comfortable, your dick will be pushing up on a brick wall. And if you manage to rip through, the scream will pierce your eardrums. I've been on both sides of the equation (I'm sure you're smart enough to figure out what I mean).
1. she's gotta be clean in there. It's no fun if she feels like taking a dump 5 minutes into it, and a chocolate dick ain't pretty either.
2. If she agrees at some point, or even if she is just considering it without giving a definitive "yes," always distract her from thinking about it by playing with other parts of her body that make her feel really good (i.e. vagina, nipples, neck etc...)
3. LOVE THE HOLE!! Lick it, message it, finger it, rub your lubed cock on it as long as it takes until you feel it loosening. For God's sake.... TAKE YOUR TIME! Íf she's on her stomach while you're rubbing your dick over it, you'll feel her start to push back on you when she's ready for you to push it in. And do it slow until your're in all the way, then start to pump slowly building up speed until she gets used to the friction.
4. Lube, lube and more LUBE. I agree with cakalac, no KY. Even hand lotion is better.
Hope the tips come in handy. Oh yeah, she'll experience alot of gas afterwards. Mostly cuz you're pushing air into her. But you don't have to share that with her till afterwards ;-) Just sit back and laugh your ass off as canon fire explodes from the bathroom. If your girlfriend never farted infront of you before, she will now. LOL