I can understand your comment in eng but it doesn't make sense for me :/ "to force the general sitemap content" ? With lastest google algorithm, google can catch the unnatural backlinks. So i don't buy backlinks in websites header or..
Maybe the keyword density does not match the terms you target. And it is links without keywords. So I would suggest only showing links to pages where the related content lies, or at least have those stickied on top. But you can encourage people to share your links which is what social media is very good for.
I know everyones been mentioned some form of backlinks, and no follow suggestions. But, have you considered looking at the websites user experience. On what can be focusing the site more to retain the visitors?
That is the first thing I was thinking. If you want to "be natural", you need to breathe some energy into it. Forum members provide fresh new content, new activity also brings natural links. I would stay away from low budget link building. But it would not be a bad idea to buy a banner ad where there is related traffic. Tunnel vision seo has never worked well with forums.