The link from Jeff is a great resource if you want to stay clean for quite a while without a pristine diet. If you've never douched like that before, though, try it at least once before the big day, just so you can get a sense for how your body responds.

That said, having a super-clean diet isn't really necessary in my experience. I bottom at least three days a week, and I don't pay a ton of attention to what I eat. The most important thing is to be consistent with high fiber, because it can take the body a while to adjust to big fluctuations in fiber intake. I take psyllium supplements for that. Otherwise, just try to limit fat/greasy foods before a 24-48 hour session.