Your Most Stylish Year Ever

"It takes 21 days to form a habit," says Jennifer Garner's trainer Valerie Waters. "The first three weeks of a new routine may seem more difficult, but once you have the process down, it becomes much easier." Create a workout plan (days and times) and stick to it. "By the fourth week, it won't be a question of whether you can or can't, it just becomes something you do," says Waters.