It's taken me two years (1yr phpbb & 1yr vbulletin) to get 600 members. In the beginning, I was trawling similar minded boards, with my site in my signature, posting a lot on those boards and then replying to almost every post made on my own board. I allowed guest posts for a while (to 'introduce' my forum to the world) and as time unfolded, repeat-guests became members. Then, I added a 'links hidden to guests' and at a different time a 'guests see only 1st page' hack to my vb powered forum.
That bumped up membership somewhat. I guess the best thing to do is entice guests by blocking a little content, give them something worthy to sign up for (extended capabilities), and just keep being pro-active on your own board AND on likeminded boards (to keep your signature popping up everywhere).
HTH