Promoting a blog through forum signatures is old school, it was probably the first thing I read about when looking into promoting a blog.
I have a few forums that I am a regular at that send me over 1 thousand visitors a day for each blog, not this blog though I've barely promoted it at all. These forums are very similar, not in content but in how the community is set up, they each have a slow, steady income of posts with huge amounts of lurkers, if I make a new thread with my sites in the sig it will be on the first page of the forum category for about 2 weeks before it gets pushed off, the view count is usually at about 15,000 by this point. Each threads reply count usually doesn't exceed 10 which means if I reply, my comment will be on the first page there as well. So I'm getting 2 weeks of front page exposure for a couple of posts.
What I've noticed about big forums with posts coming in all the time is that my posts get pushed of the first page fast, and by the time I reply to something good it ends up on page ten, it all gets lost and I hardly get any clicks on my sigs. It might be good for PR to have a link on these big forums, I don't know about that.
So finding a forum in your niche that has a slow, relaxed, steady community with huge amounts of lurkers can be very beneficial.
Blog, Promotion, Forum, Signatures, Best