How to get students to use the course forum?
I think the key to a forum being somewhat lively, at least from my experience, is a blend of the following:


There needs to be instructor/TA participation in the forum itself. This is both a boost to the "signal" of the forum.
Incentivize interaction not just posting. If you're requiring forum posting, require not only an original post, but also one or more replies to existing posts.
Start broader topics early and help keep them running. Many forums are organized by chapter, assignment, etc. which kills any participation (even if you have it) the moment people click 'Submit' on the assignment.


That being said, I cannot yet think of an instance where I was actively glad to have the forums for an online class.