I was thinking of banding together 5-10 people who are struggling to build up their forums and just make an agreement to post in each others' forums as regularly as possible. We'd each bookmark each others and post all around. It'd be a good way to build a good base for each forum and it wouldn't cost a penny. I read the rules for this forum and this seems legit. If it isn't I'm sincerely sorry, this just seems like a good way to help other people while getting help at the same time.
There are a couple of forums where you can ask for and do post exchanges. I like this one - http://www.ipostyoupost.com/ The "I post you post" is a new forum, but it seems to stay pretty active. I'am also a member there.
I'd also like to do posts swaps, as I have in the past, on a post-to-post basis, where we post the same number as we receive from others. I remember a site that had tracking for just that, and you didn't have to post on everyone's forum but the system kept track of how many posts you had made and received, to keep it balanced - anyone remember this ? Unfortunately I run an adult forum so it's difficult to find others who are willing to post.