iVillage Health has put together a workout designed by Urban Jungle chief instructor Joshua Vota:
The workout is simple, there are nine different core exercises designed to focus on the mid-section of the body, front to back! You’ll also be hitting those shoulders and working the cardiovascular system with the fast movement exercises to increase the calorie burn.
One round equals these nine exercises, 30 reps of each. Each round should take about 7 minutes, so if you are short on time, or just starting out with core exercises, this workout is perfect for you. As you get stronger, or have a little more time, add another round or two, which will leave you feeling it for several days afterward.
Always remember to warm up for about 5-10 minutes before beginning and breathe through every exercise -- don't hold your breath at any one point.
Now it's time to get to work! Ab Exercises for a Flat Stomach 
What do you think?