Here's a web site with tons of advice about anal sex:
AdviceGeek: Smart Gay Sex Advice for Men
These articles deal with how to make yourself as clean as possible before bottoming, however, I don't completely agree with the author's recommendation to use enemas (see below).
How can I make sure I won't get messy during anal sex? (AdviceGeek)
What are the different approaches to avoiding anal mess? (AdviceGeek)
How can I avoid anal mess even when I use an enema? (AdviceGeek)
Use douches/enemas sparingly. For one thing, they have been shown to increase HIV transmission and suseptibility. For another, they can irritate the rectum/colon and, with overuse, they can cause long-term problems with bowel habits. Don't let anyone tell you that it's good hygiene. That's nonsense. Nature never intended the rectum to be regularly flushed with water (let alone salt water, mineral oil, coffee, non-alcoholic beer, and all the other crazy things people have used). Use one only if you need to; if your rectum is free of fecal matter, it's not necessary to douche. All you'll be doing is rinsing out any mucus that coats the lining of the rectum; that mucus is the rectum's natural lube and protects it somewhat from injury and irritation, so there's no point in washing it away if you don't need to.
If you do need to douche, just rinse out the rectum (see the picture linked below). Avoid getting water in the sigmoid colon (or beyond). This isn't too hard to do because there's an "inner sphincter" (for lack of a more accurate term) between the rectum and the sigmoid colon which is normally closed unless you are having a bowel movement. If you stimulate the sigmoid colon...well, read on.
http://www.pennhealth.com/ency/images/ency/fullsize/19220.jpg
Stand in the tub and insert the nozzle into your rectum...relax your sphincter and try to leave it open a little--don't clamp down on the nozzle. Then gently squeeze the bulb of the douche bottle (or turn on the water if you're using the kind with a hose attachment--low flow) and just flush out the rectum.
The directions that come with the enema kit will probably tell you to lie down, fill yourself up, and hold the water in for a few minutes before sitting on the toilet and letting everything come out. Don't do that. Those directions are for the treatment of constipation. If you're not constipated, you'll just give yourself diarrhea and you'll be pooping for hours.
Keep in mind that if you're eating regularly, your colon is
always filled with feces. It's supposed to be. (Although some branches of alternative medicine advocate cleaning the colon, this is not what nature intended.) And the colon is constantly pushing that feces forward. When it gets to the sigmoid colon, which is muscular, the feces gets pushed into the rectum, and you feel the need to have a bowel movement. Unfortunately, this sometimes happens during the act of anal sex, in spite of any advance prepartions you take clean yourself out. (In fact, I have a theory that anal stimulation may in fact
cause the colon to spasm, resulting in an inopportune bowel movement.)
To minimize this possibility, and avoid certain foods for a couple of days before having anal sex, such as: fatty foods, fried foods, beans, red meat, dairy, nuts, popcorn, corn, carrots, water chestnuts, and any other foods which make you gassy or give you other digestive problems. Do eat whole grains and leafy vegetables, and drink lots of water.
Any questions?