what kind of forum is it? whats the theme?
in a general sense, here are some good ways to 'force' conversations:
- start an 'introduce yourself' kinda thread, asking name, location, whatever. they typically end up being conversations
- as noted here look for something controversial, or opinionated to discuss. post the question, then reply with your own opinion -- never fails!
- start LOTS of posts yourself (instead of just replying to existing ones). it forces variety to the board.
- ask a public opinion question (i.e; "what would you like to see on these boards?" or "whats missing from here?" something like that can both start conversations AND benefit the community)
without knowing, specifically, what your board is about or the community of it, etc, i cant get much more specific, but... the above should work on most all forums.