I've been running forums for 10 years. For me, it's all about the niche. How unique your site is and how valuable are your members to advertisers? I don't do any ad networks, all my ads are direct contract. It's hard work, but my main forum, ControlBooth.com, generates about 1/3 of my take home revenue, and it only took the last 5 years of going to conventions twice a year, making connections, getting one big company to advertise, and saying, SEE?! ________ advertises with me! Why aren't you?!
It's unbelievably hard work and discipline to maintain these contracts, but it's meant all the difference for my site and our lifestyle. I'm not rich, and sometimes things are still tight, but for the most part, I'm doing okay for me and my family. (wife + 3 boys)