Good points santana...
Having something people want helps tremendously, whether it be interesting discussion, downloads, information, etc. UBBDev went from ~3000 members to ~14000 members in ~8 months when we made the "database" section of our site to where people had to register to read. It didn't make the actual number of posters increase at the same rate, but gave us the opportunity to have a captive audience we could "sell" the rest of our site on.
Lively discussion (even bordering on arguments) get people involved, tho too much and people leave to find a more "safe" place. Balance is key.