The first thing would be "getting a good design along with a professional looking logo" as if you have good design you have more higher changes of user retention.
I would concentrate on the theme so that the users like how it looks. Then I will try to offer something unique, something that can't be found on other forums in my niche. Also, advertising and organizing games with prizes would increase the traffic and the interest.
First thing I do is work out a plan of action.
Look at who and what I want to attract, who my competition is, and look at what their community wants. A lot of sites move slow with their systems or culture and you can capitalize on that.
I have a friend who stole quite a few users from a competing gaming forum by simply having more upto date features and a more competitive environment.
Make people cry because I'm spending their precious time trying to convince them to join yet another forum. :p
I think it should be in this order:
Identify your niche, Design & Features, Contents, Advertise for members.
I think about forum structure, then spambot prevention, plugins (possibly), themes, and then lastly advertising for members to join.
I'd only start off with a new forum if there are people willing to discuss things on it.
Building a forum without a background community is like building a supermarket in the desert. It's great, but good luck with that
