Remember that in CourseForum, every page can be a discussion. If you want your class to have an in-depth discussion on a topic, it makes sense to create a new page just for that topic.
You can list all the topics on the Discussions page, but it also helps if the current topics are also listed right on the Home page, so that students see them right when they enter.
Start the page with a clear description of the topic that you'd like discussed, and then encourage everyone to add their comments using the 'Post your comments' area at the bottom of the page. Encourage them to read through other students comments first before they do!
As the discussion evolves, you may find particular subtopics or related ideas naturally evolving. If you think it warrants, you (or anyone else!) might want to create a new page just to discuss the new topic. The material from the original page can be copied into the new one. Add a link on the new page refering back to the original. Delete the material from the original page, and just provide a link to the new one.