Now that you're preparing your own meals, it's important to learn how to choose your foods wisely. In general, that means selecting foods that will fill you up on fewer calories, Katz says. Start by opting for lots of single-ingredient foods — broccoli or salmon, say — instead of highly processed multi-ingredient products, with their long lists of additives and preservatives. In other words, put down the Cheez Doodles. "They're phosphorescent!" Katz exclaims. "If it glows in the dark, don't eat it." Instead, he recommends preparing a good-for-you snack to keep in your bag for those moments when you feel peckish. Having, say, a mix of dried fruits and almonds on hand can prevent you from getting seduced by the vending machine. "It's your food equivalent of an umbrella," he says.