Well that is a community management topic you could spend a lot of time talking about .. but in short we find one of the most effective things is to be very upfront and clear about the rules and norms in your commenting community, and be very present in the discussions yourself so that your audience can see that you're enforcing and living by them. The thing is, those norms can vary highly.

Some sites or companies welcome any and all participation, even if it's critical or or even slightly off-color, but as long as it's not abusive. Others, wish to enforce stricter rules or may even decide to pre-approve all comments. Don't mean to be vague -- depends on your goals and situation. In general, though, we most often see the former rather than the latter.

Hope that helps a bit.