You can't slim down by exercise alone. It's important to cut calories as well and to know your daily calorie intake. Though it may seem tedious, keeping a food journal works — in fact, one study found that those who logged what they ate six days a week lost twice as much weight as those who only kept track of a day or less. In order to lose a pound a week, you need to cut 500 calories a day — 250 through exercise and 250 through diet is a good breakdown. Just be sure not to dip below 1,200 calories. Writing it down will keep you accountable for every bite.