It will depend LARGELY on what kind of traffic you can drive to your blog. Profitable blogs have readers. There’s no way around that. (If you are struggling with traffic, I LOVED – and by loved, I mean have read twice and will read again – Lena Gott’s Traffic Transformation Guide. She details how she went from 17k pageviews to over 400k pageviews in ten months!)
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.