Flower Darby, co-author of Small Teaching Online, has a piece in The Chronicle of Higher Education titled "The Secret of Good Online Teaching: Discussion Forums." In it, Darby offers several solid tips for using LMS discussion forums, including how to "structure the online conversations": 

Without any structure, you end up with a lot of students pulling a “post and run” — an industry term for posting an obligatory comment in a forum, and then never returning to engage with others....

So establish a few simple ground rules:

• Set two deadlines a week — the first for an initial post and the second for a minimum number of replies (usually at least two) to other students’ comments.
• Provide a rubric or a checklist (try using the rubric tool in your campus LMS; it really speeds up grading) to clearly communicate the criteria for success: How long should a post be? Can the style be informal and conversational? Is a scholarly citation needed?
• Discourage students — explicitly — from posting “Yeah, I agree” kinds of replies.

Check out the article for other tips for using discussion forums.