Followup: It depends on what you're building the forum for... If it is for actual people communicating/building a community, my tips above might help. If it is for SEO/keyword sniping, hiring trained forum posters might do the trick
Paid Forum Posting is good for SEO/keyword but it's also good for creating/sustaining activity in your forum (when just a few of your friends just talking to each other doesn't cut it)
My take on this is that it takes a forum webmaster to understand a forum webmaster. My piece of advice for start up forums is to exchange posts with another forums. The Admin Zone is the best place for this sort of deals and I heartily recommend it. Best of all, it is free. Regards, George