Cammi has written a blog below with some advice as to how to get buttlovin' from your woman.  I cannot advocate her recommended method.  There's no way you can give a girl surprise butt sex without it hurting.  You push a big puffy purple dickhead past that rosebud unexpectedly and she's going to tighten up like a monster.  Some dick head pussy juice is also no proper lubrication of anal.  Jamming the stinkhole full of love hammer without express permission is also a violation of trust.

You should get your girl used to ass play before you jam the johnson in there.  Once she's used to something up there, she'll be more receptive to the idea.  The obvious start is using one of more fingers up the butt while engaged in other sexual discourse. 

Try starting with the "carpet munching bowling grip."  Women enjoy getting oral sex.  Next time you go down on your woman, but your fingers in her primary love hole.  After she's worked up, add a finger to her ass.  I prefer to use this method as an exercise to strengthen my bowling grip by putting my middle and ring fingers in her cooch and the thumb up her ass.  This gives you some traction on the thin wall of skin between her vagina and fudge tunnel. 

Another friendly way to introduce some anal intrusion is during doggy style sex.  The asshole is right there when you're going to town and just asking for it.  Initially just moisten up that thumb and press on it.  You don't really need any penetration for there to be extra stimulation.  Once she's accomodated to it, then push it in.  If you turn your wrist at the right angle, you can have your fingers on her ass, thumb up the poop chute, and use it as a grip to improve the pumping.

After she's become used to having something up her ass as part of sex, you propose the idea of anal.  It's best to propose this shit on your birthday and as Cammi suggests, after a drink or two.  You need some disinhibition to be going on.  Make sure to get her worked up before you push a man sized dick in her ass.  I suggest a vibrator.  Get her (or you) buzzing that clit with the vibrator as you work one and then two fingers up her ass.  Move those fingers around a lot, loosening everthing up.  You'll be able to tell when she's ready when her asshole isn't squeezing your fingers too much.  Make sure you're dick is properly lubed.  There is no such thing as too much lube during anal.

After you've got your dickhead in, pause.  If you've got a man sized organ, this'll be the biggest thing she's had going in through her out door.  I usually wait until she starts moving against me as a que.  Timing is everything with ass love.  After she's had an orgasm, it'll be harder for her to tolerate your dick up there.  I can stay hard for a long time after busting a nut, so I usually just crack one off before she comes and leave it in there, moving some until she comes herself.

I actually don't prefer anal.  It's a nice variation, but I like nutting in my wife's pussy as much as anything.  I hope that this helps your brown loving strategies.