IMO... forum posting just for the purpose of SEO is not a very good use of time. For the same amount of time, you might as well comment on dofollow blogs, write good content, or write linkbait content.
Forum posts are usually considered very low importance by the search engines (especially because it's also a spam technique)... your forum profile might become a PR3 page, but that's after spending a lot of time posting on forums. It's just not really worth it.
But if you're going to do forum posting anyways, I provide some tips here:
http://www.v7n.com/forums/seo-forum/...sting-seo.html