I'm somewhat okay at getting new members, though I feel it mainly depends on the niche of the forum. General discussion forums as well as promotional/admin forums seem to get more members more than specific niche forums like gaming forums.
Since I hide the core aspect of the forum from guests, the codes, and skins, I get new members pretty consistently. People register for what they want/need, and a lot of people want/need codes or a new skin for their board.
I am working on a new forum. I would start by hiring paid posters to seed content, and once I have some content, I would be involving in post exchange and buying posting packages. I believe once I have a lot of posts, it would be easier to attract organic activities.