Female pleasure from anal sex is maximized by taking your time getting there. It is all about the build up, at least for her.
The main things that are important are to get her really aroused first and to go slow. Lots of foreplay of the standard sorts, plus vaginal sex is excellent foreplay for anal. Then spend lots of time spent getting her ass ready for penetration. I mean LOTS of time, and LOTS of lube. The more you caress her and tease her with your fingers before you try any penetration, the more she will enjoy it. And the more time you use your fingers to penetrate her before you try it with your cock, the more she will enjoy it.
Ideally, she should be deeply aroused and open enough to let your finger or thumb slide in fairly easily before you start with your cock. Wait until she is pushing back against your thumb or finger, urging you on, before you try to slide your cock inside her. It may be hard to wait, but you basically want her begging you for it before you move into her. If you wait until she is in that state -- highly aroused and dying to have you take her -- it will be better for both of you.
Then, when you enter her, try to go slow. Let her set the pace, watch her reactions. Give her time to adjust to your size as you push inside her. I find that while it can hurt a bit at first, I quickly adjust to his cock size as long as he goes slow at first. If he tries to go quickly all the way in, sometimes I cannot help but clamp down, and that makes it painful.
Remember -- if you go slow and use lots of lube, anal sex should be virtually painless for your partner. And once she gets going, you may be able to go as hard and fast and deep as you like (I love it that way, but it takes time to get me ready enough physically). BUT you must always listen to your partner and not force it -- is she isn't ready for it or up for full penetration or thrusting, then back off and try another time.