I seem to suck at building them from the ground up. All the email lists I started or co-founded went dead or are tiny and essentially stagnant (not growing, at least not significantly). But I have a decent track record for jump-starting conversation and increasing membership at a later stage. Two basic rules I followed:
1) Greet people warmly at the door. If they introduce themselves, say "hi", even if they haven't given you much to go on. My observation: People greeted warmly tend to join the conversation. People who are ignored at that point often never, ever post again.
2) Don't tolerate "orphaned posts". If no one has replied in X time (which would depend in part on the forum in question, but I used to use "3 days" as a guideline), then reply to it even if only to apologize for not knowing what to tell them. If at all possible, try to give them something useful/meaningful in your reply. Even if all you can do is acknowledge it, on an email list at least, simply replying will bump it back up and increase the odds someone else will have something meaningful to say.
Those were my two golden rules. They did wonders for several different forums I joined later in the game. In two cases, I was a moderator at one time or another.
If you get any good tips for starting one from scratch, I'm all ears. I can't seem to get the hang of that one.