Most forums and blog posts are hardcoded as nofollow, so don't expect any ranking improvement or links from your signatures. But as JackTB mentioned previously, if you deliver high quality content (i.e useful tips and info) then people will click on your signature just to see what is it that you're doing/advertizing.
Google made an algorithm change that now makes link sculpting impossible, as nofollow links wont give more joice to normal links. But this only happened a few months ago and as the SEO industry is prone to urban mythis (eg. keyword density importance in an era of LSI, or importance of keyword meta), it will be some time before webmasters realize that it makes no difference if the forum and blog posts are nofollow. So, it's best to focus on high quality content on your site, and in your comments. The point is to be valuable to the community, and people will start spreading the word for your expertise... and that's what you ought to be after, building a brand around your name.
Offer great content and people will come to your site.
by happygeek: Self promotional link snipped
Right, I agree. Your forum signature will have no effect if your posts have no value or is just one of the many "thank you for the info blah blah" posts. Adding value to a thread is more important than just being an nonsense poster.