In a forum on WebmasterWorld , one thing Markus Frind suggested for those wishing to be successful as AdSense publishers really stood out for me:
“Do not enter markets with a lot of competition monitized via adsense. Try and undercut paid content markets by offering a free service, or better yet create your own market.”
So, there you have a great formula for success: choose an industry that charges fees for access to info/content, and offer it for free, paid by the contextual advertising.
Newspaper Industry: are you listening?