So, I think I'm ready to post this. "Cleanliness is next to Godliness" is an old, old adage. But, from what I can gather, this is an important step in preparing if you even think anal sex might be on the agenda.
Nothing is worse than, as one Faithful Reader stated, pulling out and "finding your dick full of you-know-what." So, if you're going to be on the receiving end, get clean. Douche or, some folks use enemas.
The biggest fear most have (women and men - but particularly the guys) is that it will hurt.
And use lube - lots of lube. Some guys produce a lot of pre-cum, yet it may not be enough. So, lube up really well.
Unless you are in a completely monogamous relationship, and you know for certain neither of you have any Sexually Transmitted Infection (STI), always, always, always use a condom. Which means you must choose your lube carefully to make sure it is compatible with latex.
Once you've entered, no jack-rabbit pounding.
At least not until your partner gives you the okay."There are also some books that can really help you understand and enjoy anal play. Joe Kort, PhD suggests The Ultimate Guide to Prostate Pleasure: Erotic Exploration for Men and Their Partners by Charlie Glickman, Ph.D, and Anal Pleasure and Health: A Guide for Men, Women and Couples by Jack Morin, Ph.D."
Finally, French Patrick provided another great resource. This article on Quora provides a great list of dos and don'ts.