Write interesting content and write every day.

Have more than one site. The more sites you have, the more ads have the potential
to get clicked.

Set up channels and monitor your statistics across all sites. If one site is performing
poorly, consider changing the content or the ad layout. Experiment!

Change the ad layout and notice which layout works the best for you. There are
many different layout options and there's no way you can know going in which
one will perform the best.

If you're blogging, writingup.com has a list of profitable topics. They state that
you don't have to write on those topics but you might give it a try or at least plug
some of them in to the search engines.

Google different key words and phrases and notice how much and what kinds of content and ads you see.

Don't be afraid to ask questions.