The following advice are found in Clay Shirkys excellent book Cognitive Surplus.

Start small (limit audience, limit scope)
Ask “why?” (Different motivations of the users. Autonomy, Competence, Generosity, Membership + more)
Behaviour follows opportunity (Behaviour is motivation filtered through opportunity. Reward motivations. Opportunity w/o motivation gives nothing)
Default to Social (“Triumph of the Defaults”. Social value should be default.)

A hundred users are harder than a dozen and harder than a thousand (intimate scale -> public scale. Being a participant in midsize group feels lousy. Once culture is established whether good or bad it’s very hard to change. The key is to recruit the first dozen of users to embody the right cultural norms)
People differ, more people differ more (Average is useless, behaviour of most -> least active differs a lot. In large systems a core group and a peripheral one emerge. Offer different level of involvement for different users! Make the size of smallest possible contribution small, make the threshold small!)
Intimacy doesn’t scale (Consider audience vs cluster patterns.)
Support a supportive culture (Let users enforce rules, culture etc. Don’t push)

The faster you learn, the sooner you’ll be able to adapt (continual learning, change the opportunities on offer – not the users)
Success causes more problem than failure (Don’t plan too much – Try & adapt. “If you want to solve hard problems, have hard problems”.)
Clarity is violence (Solving the problems as they arise means not putting process in place until you need it. Groups tolerate governance only when enough value has accumulated to make the burden worthwhile. The rules has to follow, not lead)
Try Anything, Try Everything (Encourage experiment, we can’t see the full value from the start.)