Before I met my boyfriend of almost 2 yrs now, I always topped with the exception of 2 occasions where I tried briefly. I can say that those two times that I tried bottoming lasted less than 5min because I was not relaxed and it was extremely painful even though the guys were average sized. That being said, I thought I would never bottom again until I met the bf. He was like me in the sense that he always topped, tried it once and then never again. We fell for each other fairly quickly and I was very much in love with him. I knew we were both tops, but I wanted to feel that closeness with him so much that I opened my mind to the possibility of bottoming again. I had to prep myself mentally as well and physically. If you are trying anal sex for the first time and you are on the receiving end, your state of mind is even more important than the other prep stuff. It was my experience that if you shit, then use an anal douche and flushed yourself out a couple times you were good to go, provided you are not cramping or having other GI problems. And don't eat anything crazy that day if you know you are going to have sex, keep it light. It's going to hurt in the beginning, but if your partner plays with your hole and uses plenty of lube it will lessen the pain, so alot of foreplay is recommended. Let him eat your ass and tongue fuck you first. Then, progress to using a finger. Let him explore you gently, and find your prostate (about 1-2inches in) and massage it - it is pure exctasy, trust me. Keep talking through the whole process letting him know what you like and where to keep touching you; let him know to stop if you feel pain or slow down until you've relaxed your hole enough to continue. Sometimes you need to take a little break, so play with his hole, or dick and then come back to playing with you. Let him rub his cock along the outside of your hole, and maybe alternate with finger penetration. When you think you are ready for his penis, the best position to start off is you lying on your side and him behind you. Take it very slow, especially if his cock is on the larger side. Don't be intimidated if he is very thick or long, just talk him through it and let him know when to stop, remember if he cares about you he wants to please you, not hurt you. I was very nervous about having sex with my bf initially because he has a very thick penis, probably about 6.5-7 inches around, and that's alot for a beginner like myself. But now, almost 2yrs later, after lots of foreplay, communication and lube, I enjoy every inch of him. I can even let him fuck me hard, and enjoy every minute of it. I can take him missionary now(which used to be impossible because the angle of penetration is really intense)and it's one of my favorite positions cuz I can watch the expression on his face when he cums inside me; the other is when I'm on top and I ride his cock, but I can't do it very long cuz his dick is so thick that it presses on my prostate and makes me cum within minutes. Everytime his cock is inside me, my cock is leaking with a constant stream of clear pre-cum, that's how good it feels. I almost can't get enough of him now. We live together and are in a monogamus relationship so we fuck bareback and I let him cum in me everytime - which he loves. If you are just having casual sex, always wear a condom.
One day I only hope he will let me fuck him, until then I'm not complaining! Hope this helps you -- good luck.