As always, this matters more to some people than others, but in this case it's a more popular issue than with just about any other kind of sex, for obvious reasons. First, having bathed more recently than the last BM is a good idea.
It's a good idea to have a bowel movement an hour or so before, to ensure that nothing is encountered during any penetration (unless you're into that, I suppose), and then bathe the area.
Other than basic cleaning like that, oil-based lubricants are a great insulator for anal contact. They tend to cover up any potential (or simply feared) smell and transfer of icky stuff. If you use Vaseline/petrolatum, for example, the odds are that a simple wiping off with a dry cloth will result in finger/penis/whatever seeming to be just about perfectly clean. Again, this may not matter to some people, but it's worth mention because it is crucial to others.
To some people, enemas may seem like overkill, or even to be grosser than not having one before anal sex anyway. But they certainly do bear mention, in case this sounds like a good idea to you (feeling comfortable with things like cleanliness are potentially very important). This is especially worth considering in actual penis penetration, since one gets a lot deeper than even the most dilligent bathing process is going to be able to clean. But it is still not a really common practice, even for actual penetration. Of course a few people even find enemas to be sexually exciting themselves.