If it hurts, it isn't be done properly and the receiver isn't prepared. I find I have to have an orgasm or many of them before I am anywhere near ready for full anal sex. (I can have anal play, but my "flower doesn't open" until I've had at least one orgasm, preferable many of them.)
I find I have to be relaxed, in the mood for anal sex, have been playing sexually with my husband for a while, usually had some PIV, and oral sex for me before attempting anal is a MUST. My Man also discovered that if I have a phallic vibrator or dildo in my vagina I am much more likely to relax for anal sex. Most women are accustomed to and certainly enjoy vagina; penetration and this, along with the added G Spot and A Spot stimulation the dildo can provide during anal sex really helps!
Good lubrication is essential. When we've had oral sex and some toy play I've come, and we've played and I've come some more, and I've give him some head AND we've had some PIV, then and only then will we lube up and start any anal sex. My Man always takes it slowly and gently at first, even though we're pretty experienced with it. It may take 5 or 10 minutes (I don't know, I'm not looking at the clock) before actual complete penetration is achieved. This is not a "ram it in and go" activity!
The receiver has to be completely prepared, totally aroused and wanting the sex for it to happen without discomfort. It shouldn't
hurt at all, although the sensation is intense.
Take is slow at first, take it easy, STOP if she's in pain, use a lot of lube, make sure she's had some orgasms, fill the vagina with something, and good luck.
Oh, and to answer your question, our favorite positions is with me virtually flat on my belly with his legs surrounding mine from behind, or on our sides. Cowgirl doesn't work for us, but every couple is different.