The best way to get the ball rolling on a new forum is to tack it on to an established site. If you have a decently sized newsletter / list, blasting out an announcement about it will help a great deal.
If you're starting literally from scratch, you're going to have a tougher time getting it jump started. Do as much promotion as you can, and while paid posting IS an option, I've only had lackluster results with it for the most part. If you do go the paid poster route, I'd suggest finding a team to do it with multiple usernames. If you go to DP and individually hire a bunch of retards, expect frustration.
Personally, I much prefer the blog-then-forum combo. Establish the blog and get a list going, then do the forum. Depending on the scale of the project, you'll probably want to be active in your forum (at the very least in the beginning). Blog+newsletter+forum = very close contact with your users, great trust building.
Edit: This also depends on the size/niche of the project, but you could offer some form of reward or incentive for the user who refers the most new members. I've done this before and seen pretty decent turnout, but you'll want to make sure you know how to check the validity of the 'referrals,' in the event some douche wants to game you. If you don't want to give out anything of value, offer something silly like a badge or special title - or even a mod position, which will cut down your work.