Search engine optimization is a key factor for a forum's success, and you should have in mind that the most forum software packages (eg. phpBB) has no good setup for this, so you may have to install some MODs or hack yourself your forum. What you should check for sure is to remove the session variables in the urls, for the search engine bots.
Forum signature is helpful in building backlinks to your site and they can also be good sources of direct traffic overtime. Whats important is on how well you participate in them. Be sure to be constructive and helpful while participating actively.