Give the group periodic summaries of what has been said, especially before moving on to a new topic. Find a good facilitator. Ask around the community. People in other organizations may have seen a particularly good facilitator in action. Be certain to ask for references.
Some facilitators are more competent than others.
We may have a good recommendation for you. Consider your own staff. There may be someone on staff who could facilitate the dialogue. If this is a possibility, do some role playing to test him or her out. Work with your facilitator ahead of time.