H20
 

Water is the medium in which most cellular activities take place, including the transport and burning of fat. In addition, drinking plenty of calorie-free water makes you feel full and eat less. Drink at least 1 ounce of water per 2 pounds of bodyweight a day (that's 100 ounces for a 200-pound person). Keep a 20-ounce water bottle at your desk, fill it five times a day, and you're set.