Document everything, and not in email!
Use something with a history. I've been tempted to use Google Wave for tracking a project's "Development" (changing requirements, interpretations, etc). A wiki will work too but has a higher barrier to editing and may be updated by fewer people. Campfire is also a good methodology.
The new methodologies (Campfire/Wave) are essentially recorded chat logs that you leave open all the time. Campfire has no way to "Promote" important decisions, I think they'd get lost in the general conversation--but with Google Wave and Wikis, you can continually trim out the irrelevant or old information. Wikis will give you more ability to reformat the new.
Actually a combination of Wave/Wiki might be best. Just use the wave for daily IM type talk, and pull important threads/decisions onto the Wiki.
Some of the practices in XP (Agile) help here as well. If you go FULL ON xp (not just calling your daily meetings "Scrums") you will find some important help such as tracking requirements on cards that are constantly updated or having a customer on site to answer important questions. The whole idea of XP/Agile is based around the fact that requirements change and those changes need to be tracked and that they effect the release schedule.