If you aren’t part of an ad network then you are constantly searching for new companies to advertise on your site. This one was always a pain in the ass for me. I wanted to spend time making my site better, but if I ignored selling my ad space then I wouldn’t have money to continue to make my site better. You can join an ad network, but that means you are giving up a percentage of the revenue. Work hard to only get 50% of the money? Bleh.
Perhaps the most popular network for placing these types of ads is Google AdSense. With this program, you do not need to be in direct contact with advertisers; you simply place the banner on your site, Google chooses ads relevant to your content, and your viewers click on the ads. There are countless similar programs available if you find that AdSense doesn’t work for you, such as Chitika, Infolinks, and Media.net.