Hey Dan,
The most important thing is to have lots of traffic coming to your website and some social proof. That will make the job a lot easier. I get these banner ad networks contacting me all the time simply because they see my blog getting shared all over the internet.
The only reason I don't use them is because they post general banners that are not targeted for my blog readers, so they end up making my blog look spammy. But I'm in a niche that doesn't rely on banner advertising that's why, maybe these networks will work for your website.
But the main thing is to get a lot of traffic in order to attract advertisers, that's it. A simple solution for your problem is to submit your website to ad network sites like:
-BuySellAds.com
-sharethrough.com
-komoona.com
-frontend.sekindo.com
-openx.com
Simply submit your website to one of these websites and if you get approved then you'll start generating some good ad revenue. It's much simpler this way, unlike hunting advertisers by yourself, these sites will do all the hard work while you focus on the marketing.
Hope that helps