Decide on a few simple rules RIGHT NOW and enforce them consistently. By few, I mean like 3-4 tops. Any more or if they are complex, and they will be totally ignored by your users. IF you make the effort to enforce them consistently up front, it will pay off majorly down the road. If you let things get out of hand and people are used to it, it will be virtually impossible to get things under control without it effectively killing the forum in the process. I started my forum 15 years ago. Here are my rules,
http://www.twtex.com/index.php?page=rules
1-4 were the original rules and 5-6 were added much later. Enforcing these upfront was a lot of work originally, but it paid off because after a year or two, my users were self regulating and helped regulate new members. Now, even though I have 20K+ members, I do very little moderation work and folks appreciate the overall tone of the community.
Be patient. New users won't come crawling out of the woodwork. It takes time. I was fortunate that when I kicked off my forum, I had around 400 emails I had collected in the years prior from people I had met while riding motorcycles. When I launched the forum, I followed that with emails to all of those people letting them know about the forum. I would also visit places where people would ride or hang out and tell them about the forum. I even had business cards I could hand out with the forum name, site address, my name, and an email address. It took about a year for it to start picking up speed. Once word got out, people started coming in faster. At first, I was doing most of the ride/event organizing and starting threads. Eventually other members started taking over planning of rides/events and starting threads. Now it pretty much runs itself.