Banner advertising and direct advertising is hard work. At a simplistic level you need to find the advertisers, approach them, sell the value of advertising on you site and negotiate terms. Many entrepreneurs will be able to come up with an initial list of target companies but often the hardest part of the sales cycle is identifying new prospects who may be interested in advertising on your site.
Surprisingly, Google AdWords maybe your best prospecting tool! This may seem like an initial paradox as Google AdWords is usually employed so that you do not have to worry about direct advertising. However, by looking at what adverts Google displays you can get an insight into the companies that are advertising online and targeting an audience that visits your site. i.e. The advertisers Google serves for your site can be approached directly as direct advertising clients.