Pay attention to what you're buying,
Is it fresh?
Does it have a long shelf life?
Is this natural?
While all of your food does not have to come straight from the earth, it's good to buy more fruits and vegetables than things that come in cans or packages that maintain a longer shelf life.
Not all foods with a long shelf life are bad, i.e. peanut butter, but make sure to check labels and ingredients next time you go grocery shopping and get more fresh produce!