I'm batting a thousand for getting regular anal from every long term girlfriend and my wife. IMHO, anal is all about psychology, not just in order to get it, but for you to enjoy it, etc. For better or worse, i'm addicted to ass, and it's just a non-negotiable.
I won't go into my long thought out theories, but here's my advice:
1. Tons of Oral! Go down on her for half an hour ahead of time. If you aren't good, get good. If you're going to be banging her backdoor you owe her some real orgasms. During the oral always include some rimming, fingering her ass, etc. The goal (aside from her immediate pleasure) is to associate good feelings with ass play.
2. To go along with #1, praise her ass! Hopefully she has a sexy ass and this will be easy for you! Don't lie though - if its not what 'society' considers sexy (either applebottom, big and round or firm and petite) you need to find a particular pose, clothing, position that looks hot, and make sure you let her know! Girls have all kinds of hangups about their bodies and doubly so for the butt, especially if you are getting in there!
3. When want to do the deed, make sure she is ready, but don't make it clinical! It's not a pelvic exam, its sex. If you've been going down on her and rimming properly you should already have it well lubed up. If u need more, discreetly lube your cock, not her ass.
Now, when its time to penetrate, make it URGENT but CONTROLLED. What do I mean?
Your urgency should turn her on. You are fucking HER in the ass, not fucking an ass attached to her. You want her, you are going to take her, and she should know it. It's not about force or power or 'roughness', its about passion. Your grip on her arms should be stronger, your kisses more intense, your bodies should be pushed together tightly. Grab her hair, squeeze her tits, etc. The goal isn't pain or control, its expression of animal attraction.
Imagine you are shooting a love scene for a movie, you want it to look like you are all over her, and when you penetrate, everything except your cock should give the impression you just slammed it in. OF course, you are in complete control and your cock is moving in as slowly as needed.
If you're doing it right, at this point you might only have the tip in, but all the rest of your action makes her think "WOW HE IS SLAMMING ME SO HARD... AND IM TAKING IT! IM THE BEST FUCK EVER!"
When you are all the way in, use your mouth and both hands to stimulate as many erogoneous zones as you can, and when you start really fucking, rub her clit just how she likes.
Grunt, groan, yell, shout, scream, wimper, shake, scratch, bite... ham it up (try not to laugh) all the while being very careful! When you are finished, go right back to praising her and lots of cuddling!
You mentioned she lets you do anything sexually, I might ask... how submissive is she? IF there is not already a dominant/submissive or sadomasochist aspect to your sex life, this might be the place to introduce it.
She may never really ENJOY it directly, but she can enjoy that she is able to make you feel so good. She can feel proud at taking it, turning you on so much, how tight she is, etc. The brain is the real sex organ. Really think about why you like fucking her ass, and a lot of the reasons have nothing to do with the physical act itself.
Oops longer than i thought! Well I hope it's helpful.
