Google lets advertisers bid on how much they want to pay per click. When someone clicks on your Google AdSense ads, you may only make a few pennies. You can filter out the low paying ads by going to http://www.adsblacklist.com/. This is 100% legal.

You want your articles to filled with high paying keywords that have the least competition. Use the Google Adwords Keyword Tool. This page will tell you what the average advertiser is paying for a specific keyword.

Enter the keyword or phrase into the keyword tool.
Check the "use synonyms" box.
Select "Cost and Ad Position Estimates"
and enter $100 under the amount you are willing to pay per click
Click "Get more keywords"
wait a few seconds...
Click "Estimated Avg CPC" twice

You now have a list of which words Google Adsense ads pay the most (on average).