I am going to forewarn you, some of the things I mention could be against AdSense guidelines. Their rules change all the time, and you should always consult with them before following the advice of others.
I make a pretty good amount from AdSense each month. I set up my first true AdSense site sometime in 2005 and have enjoyed the passive income it brings me each month. I have now set up a ton of sites that use AdSense for income.
Here are some of the things I have learned since that time:
1. You must always experiment with your ads and content. I know that doesn’t sound “passive”, but once you get this down in your first couple of months, you can just leave it “as is” and collect checks.
2. Do blend your ads with your content. I try to always have a test where my content matches the same css (as close as possible) as my AdSense ads. I use the same font, same size, make the text color the same, the background color the same, etc. I even match the title of the AdSense ad to the color of my text links I am using on the site. You should still change it up and test it out to make sure your surfers are clicking as much as they can.
3. If you’re using black text on your site, try changing it to a lighter color like #333333 instead, also change your ad text to this color. The lighter color will almost always make your title of your ad unit stand out and “pop” more, making it more appealing to click.
4. I used to place my ad units right under the H1 and above the content, but since AdSense frowns on this, I now float left my ad units within my content. My content now wraps around the ad unit. Float left seems to work better than float right.
5. I almost always use 336×280 ad units. You should play with those and 300×250. Those have always worked out the best for me.
6. Only use one ad unit, regardless of what anyone tells you. Matter of fact, just test it out yourself and see where you make more money. What I have found is, the more ad units you put on your page, the more customers have to click. However, they could be clicking on the lower-paying ads in those other ad units. When you display one ad unit, you display the top-paying three to four advertisers.
7. Test out a 300×250 image unit ad next to a 300×250 text unit ad. You cannot place a normal image next to an AdSense ad unit, but you can set two AdSense ad units next to each other. Just make one image only and the other text only—same effect.
8. On the larger 336×280 test ad units, the URL of the ad sits next to the title of the ad. Both are underlinked. Try making these both the same color. The effect is, a same-color, lengthy, underlined piece of text that draws the user’s eye to it.
Hope this helps. I might post more on this later next week.