At one time a bunch of links to your site helped whether you just put your URL or you use keywords to describe your URL in the hyperlink. These days I believe if you have hundreds of forum links, it may help some but the engines no longer put as much emphasis on that backlink as they do backlinks from different sites who just link to your site because they like it.
IMO, the best way to get the most out of forums is to do what others have said... post helpful, useful information. People will visit your site because you have developed some credibility and people are naturally interested in what your site offers.
There's nothing wrong with using a forum to promote your site and I know that's why a lot of people are here. (I think the cash prize helps too
) That's fine. As long as people continue to post useful information that is relevant to the topic and not just say "visit my site!" (AKA spam) then it's OK.
A forum is a win-win situation. The owner (me) benefits from the content contributed by the members and the members benefit from being able to promote their sig file.
But you no longer get much of a benefit from having the same link all over the place (sig files). Yes, they help some but overall to get the most benefit from a forum is just by people clicking your link in your sig... not from the search engines giving you credit for having the same link in multiple threads.