Only one rule: Any suggestions for improvement (criticism) should be made privately to the idea contributor.
Create important categories to focus on brainstorm and create categories of the membership problems/solutions such as
New Members - how to create better first impressions for new members,
Existing Members - find out why existing members dropped off
Modify the home page to make it more welcoming
write a group mission statement that is engaging for members
find a way to publicly recognize contributors (one example is the point system used by stackoverlow)