It's easy for celebrities to stay so thin, because most of them have both a personal chef and a personal trainer. Think about it, their food is always healthy, and tasty, and for 1-2 hours a day, they have someone kicking their butt in the gym. Who wouldn't stay thin that way. As for getting a flat, firm tummy, my advice is to increase cardio exercise, and do a lot of stomach crunches on an exercise ball, all while eating healthy, and cutting out sugar and salt.