All you need to get a sexy stomach is just five minutes of tummy toning exercise a day. The best and most efficient recommended exercise is the 'bicycle abs' to target the six pack. It also helps to tone the waist. It is most effective because you're continually moving, so muscles never rest and you get the best effect. Checkout following videos:
Ab Crunches Video
Swiss Ball Crunches Video
Roman Ab Crunch Workout Video
Also checkout this video for more information.