Don't eat anything your grandmother wouldn't recognize as food.
Michael Pollan, author of The Omnivore's Dilemma and In Defense of Food (and subject of the Enlightened Cook column in our June 2007 issue), speaks at Google's headquarters about why we should be eating locally grown, organic, unprocessed foods. If you haven't read one of the books above, you should. This guy turned me into the rabid locavore food activist I am today.
It's a long talk (almost an hour), but well worth watching. What Pollan's message boils down to is: "Eat food. Not too much. Mostly plants." I especially like his advice about unhealthy snacks like french fries—you can eat them whenever you want, provided you make them from scratch yourself.