It can get very, very messy, just from my personal experience I've found that: you should "empty the tank" so to speak a few hours before the actual act. Be clean. Very. Very clean. There can NEVER be enough lube, it's going to hurt at first, possibly/probably the first 5-20 minutes, don't augment the pain. Really all thats left to say is take is slooow, Theres no need to rush.

As the previous poster said, 'empty the tank' the best you, yourself, can. Also, enema's help a lot. You can buy a kit at your local drugstore usually, not all have them but some should, I only paid $8.99 for mine. It seems/sounds deeper/more painful than it is. I was afraid to do it first but it actually felt nice to cleanse myself out. You can read up more on it for how to do it. And of course as said before, lots and lots of practice (fingers first, toys, etc) and when you do do the anal sex with him the first time make sure he knows to go SLOOOOOW!!!! Use tons of lube, don't care about wasting any there's no such thing as wasting lube for anal sex. Use the whole damn bottle for the first time if you need to. And it may help to have a butt plug up there for a little while before he goes in there, just so you're loosened a little more and more flexible. And it will definitely help to either finger your clit or have a vibe there while he's in your back door, it will add more pleasure and help to take away from any pain. Good luck :) *Also in reference to an 'embarassing situation', it's normal if it does happen. You both should be aware of it and he shouldn't be grossed out because it's a possibility and nothing wrong if some comes out, just have some tissues/wet clothes nearby for easy cleanup. If he's not mature enough that he makes negative comments about it, he's not man enough to try it in my view.

Don't forget - your strap-on shouldn't be bigger than he can handle. YOu don't want to hurt him do you!

take a good crap and then after that try it and it will not be as messy

Do not rush! Relax! Lots of lube! You'll be fine, and you might like it! I didn't like it until the 4th time, but the 4th was with my husband and we finally did it right. I love it!

Take an enema a few hours before having anal sex. Read this post for more info: http://mflife.wordpress.com/2006/10/12/anal-sex-101/

SHIT HAPPENS!

just relax! take deep breaths, and relax your muscles. If you tense up, it'll hurt very bad!

Is this to be tried on him or on you, if for either of you just be clean, dont fret the small stuff. There is one way to make sure nothing comes out you dont want to see, whoever is going to be penetrated should use an ennema, but this could take a while to work so where is the fun in that, just use a condom and tell him to use lots of lube get some babywipes and go at it. The best time to do it is after one uses the bathroom for #2 and then bath of course. There is never a for "sure" way to make it so it isnt messy. Just make the best of it and if it doesn happen and it is true love you both can laugh it off and say, maybe it will be nicer next time.