Don't market THEM, it's multiple sites ... probably around 10 - 15 now, maybe more. I haven't counted in a while. Strict SEO, link building, fresh content. It doesn't hurt that I have 1 or 2 mature sites with keywords that ranks me on the first page in google (#1 - #9 positions) that pays very well for each click. I signed up for Adsense way back, threw it on a site and didn't really do anything with it until I noticed cheques kept coming from every month. I got serious at SEO & traffic building in the last 18 - 20 months. Revenue's been steadily improving by just sticking to the basics of SEO ... great content, keywords and link building.
OMG I can't believe what a difference it makes moving the ads above the fold. I was making less than a $ a month. But the other day I moved the ad to the top of the page, and with the same number of visitors (actually less) I'm now making around $1 a day!!