This is primarily focused on those working in an office environment, but I think the rules apply to everyone. Having worked retail in a former life, I understand that sometimes the fact that you got to inhale some food during the 10 minute break during the 12 hour work day you're on is a miracle.
Yahoo has an interesting article on the kinds of foods we should and why. I like how the article focuses on brain power and recommends good snacks to have on hand. It's unfortunate that most food vendors and vending machines only sell empty carbs that serve to spike our blood sugar and then allow us to crash. This creates a horrible spike-crash-burn cycle that many of us repeat. By continuing to scarf down empty carbs we don't get the nutrients we need and our waist lines show it!
Honestly the easiest defense against all of this insanity? Bring your own food. Breakfast, snacks, and lunch included. It saves you money and helps keep you healthy. It's not always easy and sometimes impossible (especially when you are socially obligated to lunch with your coworkers at a local eatery), but when you can bring your own food you feel much better for it - provided you packed the right food.