One thing for sure, if shorty's not very sexual or willing to be freaky, then it's a no go. Boring chicks don't try new stuff.
The thing is, you gotta change their mind on it. Make it seem more emotional and less sexual. If she thinks it's just a perverted fantasy, it's not gonna happen. Ever. They usually think it's slutty, or dirty. Some girls treat it like their second virginity so they protect the hell out of it. You gotta use that wordplay to change her perception on it.
Be ready for the excuses too. It hurts. "don't worry, we'll start slow untill it loosens up and I won't go fast untill you're ready.I'll help you relax. If you,re relaxed it won't hurt" it's dirty. "it's nothing. Sex is dirty too. A little cleanup is nothing." I'll feel like such a slut. "sluts sleep around and give it up to anyone.this is between you and me.this is our thing *and I want us to try everything together for the first time cuz I care about you(or love, depending on how thick you wanna lay it)"
*assuming you're below 18 that works well.
The surprise doesn't work well. A woman's anus only has one layer of mucous membrane so it's not naturally lubricated for that. And using natural vaginal still won't make it a comfortable experience. Get some astroglide. If you're using a condom, use ky. Astroglide is oil based so it'll break down the latex. No need to use rubber in the booty though, unless you're scared of a little dookalax. Lol.
Ummm, what else? Be real with it. Talk about it beforehand so she starts getting into the idea.
Okay, and if you eat the cooch, lick the booty hole a few times and see how she responds. If a chick isn't really heated up, they'll hate it. But if you're getting her hot, usually they'l be surprised by how much they like it.
That's all I can think of off the top of the head. Hope my little essay helps. Lol.