Allen Varney in an article at The Escapist talks about converting a role-playing game from paper to online with collaborative input from fans. He provides suggestions that could be useful in any cooperative development project:

1. Excited interest
Promote your idea. Convey why it's cool, why people should mess with it, and how they can improve it. If you can't get a dozen people excited about your creative property, it's probably not worth pursuing anyway.

2. Fast, frequent communication
After you build energy, synchronize effort. Use mailing lists, instant messaging, forums, blogs, and shared netspaces of all kinds. Use a Wiki! A collection of editable Web pages is probably your best resource. Note, though, Wikis select for deeply involved contributors. It takes so much time to stay current, lightly involved onlookers may soon drop out.

...via BoingBoing