6. You chug without thinking. What you drink affects your spirits as much as what you eat. In moderate amounts, caffeine can enhance physical and mental performance, but too much can spur anxiety, nervousness, and mood swings. Stick to one or two cups daily to dodge the negative effects. "Coffee, for example, is a stimulant, and when it wears off, you're going to feel a drop," says registered dietitian Constance Brown-Riggs, a spokesperson for the American Dietetic Association. "In terms of feeling better or having more energy, it's far better to drink water as your primary beverage." Another smart choice? Low-fat or skim milk. Dairy products contain lots of whey protein, which antidotes stress, improves mood, and enhances memory. One cup a day will induce a sense of well-being and relaxation. If you're not a milk person, green tea—an antioxidant powerhouse—also fights depression. It contains theanine, an amino acid that helps combat stress.