How do you prepare for anal sex?

We don't do any real prep beyond making sure we are very "clean" and fresh from the shower. Of course, making sure your bowel is empty (recent bowel movement) will make sure that rectum is empty and never had a problem with "residual" anything coming out...in decades of doing anal. We love it...just lube, lube and then add more lube. Then, of course, add extra lube and then some more.

I agree to this! us too

There are some things that are known about enemas and some that are not known.

We know that putting chemicals in your colon can cause damage to it, and there are even some cases of people being put in the hospital because of a soap enema damaging their colon or the hyperosmotic "Fleet-type" enemas causing electrolyte imbalance. It's also known that if you absorb too much water you can have electrolyte imbalances the other way, but you'd have to take a pretty big enema for that. Plain water has also been shown in one study to cause observable changes to the colonic mucosa, but it isn't known what that means/what effects that has yet.

If you do enemas it's best to keep them as small as possible and consider using a saline solution. Routinely taking larger enemas can potentially mess up the body's "trigger".

In general there don't seem to be many large-scale studies about enemas, so the general attitude is "be careful just in case". Not that I really blame them for the lack of studies - it isn't the sort of field that gives you respect as a researcher, and finding subjects will be hard. "We're doing a study. We want to give you lots of enemas and then do a bunch of uncomfortable and embarrassing tests to see if they have any bad effects" isn't going to have me lining up.

That said, I usually take one before buttplay because I'm much more comfortable when I know I'm clean, and I know that I'm not always naturally clean there. I try to be safe and sane about it and haven't had any problems.

Same here