1. Make some ads, be flexible. 468x60px, 125x125px and 120x600px banners will cover you. Then members can advertise the site and not just you!
2. Try a competition, if you have $5 in the bank, make a posting competition like "make 20 posts to get in the running to win a domain name". Its cheap and effective.
3. Do some self promotion, every forum, put your forum in your sig. Post banners up around work/school or around your town.
4. Find something unique. Make a super computer or something that will attract people. Then submit to digg, stumble, del.ici.ous and google!
5. Go to an admin forum like adminfusion.com and do some link exchanges. (link exchanges mean backlinks, backlinks mean better pagerank, better pagerank means more people)
your website. Look up mod_rewrite on google and try it yourself. Or i do it for phpbb3 and normal websites for $5 a page (just saying, you dont have to)
Please rep if i have helped you
and ofcourse, reply or PM me. id love to help!