To maintain a lean body, nutrition is the keyword. Along with an intake of essential nutrients, you need to also do a lot of weight lifting and exercising in the gym in order to burn the calories and fatty foods consumed. In addition to these workouts in the gym, you could also do some low intensity exercises like long walks and jogging etc. to supplement the weight lifting regime.
Since there is a lot of workout required in maintaining a lean body, it is also important to eat foods that give you energy. Diets with high carbohydrates should be taken. Supplements with vitamins, iron and proteins could also be used.