While signature campaigns are the prime focus here, there's another conventional, albeit less/ more rewarding method. Linking a website in your profile, signature or posts.
I used to do this a lot, but I've backed away from it a bit now. In fact I invented the "splinky" You hosted the image of a conventional smiley, and then you included that in your post with a link to one of your webpages. The post appeared to be normal, but you gained some advantages with Google. Google seems to have changed its rules now, so it isn't worth doing it any more.