Adsense tips

Tip #1: Don't put ads on empty pages.
When I reworked my site, I built a skeleton set of pages that had no content, just titles and some meta tags. I displayed ads on those pages, however. Although all you see are public service ads at first, the very act of displaying ads on a page causes the AdSense web crawler to quickly fetch that page for analysis. A page with good content will thus begin showing relevant paying ads fairly quickly.
If you don't have any content, then, Google will have to guess as what your page is about. It may guess wrong, and so the ads that it displays may not be relevant. You'll have to wait until Google re-crawls the site for the ads to correct themselves.

Tip #2: Don't be afraid to ask questions
If you're wondering about something, don't be afraid to ask Google. So far, they've always responded to my questions within a working day. There are two email addresses to use, depending on the type of question:
Please feel free to email us at adsense-tech@google.com if you have additional technical questions or concerns. For general program or account questions, please email adsense-support@google.com.

Tip #3: Avoid non-English characters on English pages
This one is a bug, to be honest. My surname is French, and I prefer to write it out correctly with the accent grave on the first "e". Every page on my site would then include at least two accented letters, because my name shows up twice in the footer. On some pages my name shows up two or three more times.
Normally, this wouldn't be an issue. But on some pages the presence of the accented characters is enough to cause AdSense to display non-relevant ads in French. This happens whether the browser indicates a preference for French or not.

Tip #4: Check your keyword density
Although Google doesn't release exact details as to how they determine the ads to serve on a given page, they do tell us that it's the text content of the page that matters, not the meta tags. Before serving ads on a page, then, you might want to check its keyword density. A good, free tool for doing this is found here: http://www.ranks.nl/tools/spider.html
This lets you fine-tune the page before exposing it to the AdSense crawler.

More AdSense Tips
You can find more AdSense tips in my Make Easy Money with Google blog. Here's the complete list so far:
AdSense Tip #5: Manage your own AdSense account
AdSense Tip #6: Carefully craft blog or forum pages
AdSense Tip #7: When NOT to use CSS
AdSense Tip #8: Access your console from alternate domains
AdSense Tip #9: Use section targeting to exclude stop/poison keywords from your content
AdSense Tip #10: Join the AdWords program
AdSense Tip #11: Plumb AdWords for keywords