Forum Moderators: martinibuster
You can usually keep away from the banning by contributing in a positive way to the site, by not pushing your brand or website, and by providing those links only when it is relevant.
Also keep in mind that search engines aren't the end-all-be-all. If users of these forums find your contributions to be helpful, they'll likely follow your links.
A friend of mine runs a niche music listings magazine and her webmaster is seriously ill (site is on his own ISP account, not accessible). I put up a temporary page for her on my site and put a post on the main forum for that music niche.
Plenty of hits direct from the forum, and somebody was good enough to bump the post as we had a second flush a week later. Within a few days we were getting some good long tail results but nothing on the main keywords.
I had a link to my blog on a signature for a.n.other forum where I post regularly. First few posts seemed to push my rankings up but then they tailed off again. I think the moral is moderation in everything.
The main value tends to be direct clicks to the sites though. I can't see they make much direct difference to my rankings.
But if they are sending targeted traffic my way and helping with the popularity of the page, the cumulative effect must be worth it in the end.
Just don't knock yourself out over forum links. Take them when they come along but concentrate on the bigger fish.