Rule Number One: Eat food.
Rule Number Two: Mostly plants.
Rule Number Three: Not too much.
Americans have fallen in love with super-sizing (mostly thanks to the fast food industry). Add this to the fact that many of us grew up with, "Clean your plate. You know there are children starving in ___ (fill in your choice of country)," and we eat much more than we need. Pollan suggests that westerners require visual cues to know when we have finished our meal: the plate is clean, the tv show is over. We've forgotten how to listen to our stomachs to know when to stop.
Eating should be about more than just food. It should be about our relationship with food--a cultural thing, if you will. Instead of mindlessly munching in front of the TV or (yikes!) eating at your desk, try to always eat at a table, and preferably with others--make it social. This will help change your relationship with food, and maybe with other people in your life as well.
There is so much more in Michael Pollan's In Defense of Food. I recommend that you read it--but not during dinner.