Promoting your site as often as you can is obviously the best way to get members. You often find that when a forum starts off it can be a slow process but as soon as you get a few active members then others get interested and they'll start to interact.
I think when people first join a site they're usually pretty active to begin with, then a lot of the time that activity will drop off. If the sites a forum, a site owner can only do so much to combat that. Most of the time it will take posts and interaction from other users to try and keep the community active.
For any other kind of sites, once you've got followers on board, keeping your content fresh and new, will attract members to interact, as there's nothing worse than checking on a site that hasn't been updated for a few days, especially when there's new topics out there that should be on.