Well, make it look nice. Get a nice logo and template/skin so people don't think it is boring. Make sure that you can easily read everything.
Then get some active members, people who will post a lot, because if people see other people posting, they will join.
Try and get something that no one else has, or something that makes your forum special. Answer the questions: What makes my forum different? Why will they choose me over someone else?
And advertise. Put links in signatures, post them in the advertising sections of other forums.
Some good examples of this are:
ScoreHero (over 32,000 users in just over a year)
PPT (Unique and fun, active members)