You stay on track all week, so you deserve a little reward, right? Not so fast. "One cheat day can undo all the weight-loss work you've put in during the week," warns Willow. If you devote an entire day to eating foods that don't push you toward your goal, "the only person you're cheating is yourself . . . and that's not good in the short term or long term." Instead, stick to the one small treat each day, and you shouldn't need a whole day to let loose.