A lot of women hear 'anal sex' and their immediate reaction is "NO WAY, EXIT ONLY!" Either they aren't comfortable with it, or they've had a bad experience. A bad first experience with anal sex can destroy the possibility of learning if it's something a person enjoys, tolerates, or simply can't get enough of!
Please guys don't push it! The worst thing you can do is try to force this experience on a woman; she will resent you for pushing her into it and then not find any pleasure in it because she feels manipulated into it. Anal sex is a very intimate act. It requires a degree of trust that you simply can not betray.
1. Don't rush it, make sure she's relaxed, comfortable and warmed up. Sometimes it's best to try this after you've been inside her pussy and she's very turned on and wet.
2. Relaxing the muscles. Start by slowly sliding 1 well lubed finger inside her anus, gently! Not too deep to start with. Her body language will guide you. When she's sufficiently relaxed and comfortable with you fucking her ass with 1 finger then gently slide a second finger in and repeat the relaxing gentle finger fucking. Often times, paying attention to her clit will assist in her comfort and relaxation. You have two hands and a tongue, you can easily use your other hand to rub her clit, or better yet, gently lick and suck her clit as you slide your finger into her ass.
3. LUBE! LUBE! LUBE! This simply can't be stressed enough. The right lube is essential and it may take time to find the one that works best for her. It's always recommended that you use a water based lube, but they can wear out easily. Some of the new products can make her more or less sensitive, and some even tingle, which she may not appreciate. Talk about it before, during, and after. Find what works for you both. It's not just about you, it's about her pleasure too.