Of all the things a girl might do, this is the one that is really, truly, an "it depends" type of thing. You can talk about it all day and night, I can prepare as much as possible, but there are a ton of reasons why, when the time comes, it just might not be a good ass day.
Physical stuff going on with her, you just can't help. Sometimes we can't even help it! But, maybe I can help you with the part that you can control
I can usually tell from the first time a guy and I hook up whether or not I want to ever go there with him, just from the way he behaves himself. Guys who just don't seem very in tune with me and my body don't usually get to experience this because, well, it's great that they're excited, but they're going to hurt me. Guys who seem like that's the main reason they're with me don't usually get to either because, again, the guys who act like this are the guys who are most likely to hurt me. Sure, maybe there's a guy out there that will turn into a super gentle love muffin once we get to the butt, but that has not been my experience, and baby I used to WORK, LOL!
Oh, and dry fingers... fingers are amazing, such useful and versatile things, but a dry finger repeatedly shoved up there is going to, you guessed it, hurt. And even if I wanted to, I would already be too sore. Many a rendezvous has been ruined this way

Lube, lube, and more lube are *your* friends in this situation. That and female condoms, ha!
I have to be honest, one of the best ways to accomplish this is to just make the girl feel comfortable. It is a muscle, and one that we can't totally voluntarily control, so we do have to be somewhat relaxed and trust you at least a little bit to "get 'er done". Kind of like the rest of the fun stuff, the guy who makes me feel like he's happy to be there, I mean just to be there, is the one who makes me want to swing from the chandeliers. If I can feel comfortable and sexy, I will want to do more for you, and my body will be more likely to be able to follow through. I know it's not really about us, but hey, even we are a smidge human sometimes ;-p
And once you have been granted access to the holy kingdom of the bootay, just be gentle. It has to start out a lot slower than even I realize most times (see why the excited guys and I don't mix well? lol), and it's almost like you have to just tease it in until she's all the way ready. I find that if the guy does that... slow, gentle, and not very deep while I relax around him, I'm ready for a railing much much faster, and can go a lot longer. I've tried to rush it before, and it just ends up with me having to stop before even I get off, which is not an appropriate excuse for a trip to bummertown.
No one can guarantee that it's going to happen TODAY, but if you've expressed an interest, and you come across as the kind of guy the lady can trust to be gentle, she will remember. We like making ya happy
Charlotte's butt pirate PSA of the day :-