These guys are right, OP: if it hurts, you're doing it wrong. Here's what has worked for me.
Get her really horny, relaxed and laying on her back. You want LOTS of lube. LOTS. Lube her ass crack thoroughly, play with her clit while you kind of tickle her asshole.
Lube your middle finger and slowly slide it in. But don't put it in fingernail first like you're pushing a button. Use your finger pad and sort of push and let the rest of your finger slide in. Leave it for a bit, continue playing with her clit or tits or whatever turns her on with your other hand.
Pull the finger out, lube the same finger again, repeat. IMPORTANT NOTE: If you put that finger all the way in and can feel the remotest hint of a poop in there, then this is not the time. Seriously, if she's got an upcoming shit, this will be much more difficult for her and messy for all involved. Generally not a happy experience.
Anyway, keep that finger lubed, and repeat the steps above until she's relaxed and can take you stroking the finger in & out of her ass. And of course, do whatever is necessary to keep her aroused while you're at this.
Next, pull out, keep refreshing the lube, and go with two fingers. As before, push in with your finger pads, not with your nails.
Check for yourself, but I've found the circumference of my dick is about the same as the 3 first fingers on my hand. So I know if we've got 3 fingers in there, she can take my dick. Now lube the hell out of your dick, put in two fingers to relax her, pull them out and ease your dick in. If found it kind of helps to shake or "vibrate" your dick and go in slow and easy. Don't just jam it in there obviously.
Go in about 1/3, stroke a little but for God's sake, don't pull out! Pulling out will shut everything down again and you'll have to start over.
Go in a bit further, stroke a bit more, and boom, you're butt-fucking. <More to come, comment too long>
