I second the recommendation of Google Docs, as it lets both of you work at the same time on the same document and saves a lot of emailing back and forth. E and I have also done it with back and forth emails, just sending each other bits and pieces to add or change, but where that falls down is if your co-writer makes changes in a section and you've been editing it too - at the same time. You can't just cut and past her/his changes in without losing yours and must pick through it all. Google docs saves that problem and also has a revision history in case you don't like a change your partner makes (or vice versa).

Other than that, my best advice is to stay flexible about your ideas and IM is fantastic for brainstorming and compromising.