In the words of Michael Pollan:

"Eat food. Mostly plants. Not too much."

Check out his books.

Eat food = eat real food, as real as possible, as unprocessed as possible. Shop from the edges of the grocery store. The middle of the grocery store is where the processed foods are.

Mostly plants = just that. Not ALL plants, but mostly. If you eat meat (I do), the animal from which it came should have eaten plants, not byproducts, not hormones, not other animals. I buy my beef from a local farm. The slaughterhouse is also local, and as humane as slaughterhouses can be. It's a small one, not an industrial one. I've met my future steaks in the field, eating grass (and I'm okay with that; some people may not be okay with seeing animals pre-burger). The same for eggs and other meats--local and well-raised. I grow some of my own veggies, and do my best to buy local at farmers markets. I do buy regular veggies from the supermarket to supplement when I need to.

Not too much = once again, pretty self explanatory. Eat slowly. Savor your food. Overeating has alot to do with eating too fast.

I find these guidelines be be very helpful.