A lot of hard work. It takes a life-long commitment of proper diet and fitness. For some, it will require being more strict (since some ppl gain weight faster than others - different metabolism among other things) but it can be accomplished. It can take months of hard work before beginning to see results which is why so many give up along the way. It helps to keep equipment around that can measure you body fat, metabolic rate, (like a
http://www.tanita.com/ scale) etc so you can measure your progress even if it's not immediately noticeable by the naked eye.