Here is a great ab / core routine I did with my client yesterday. It consists of 3 exercises performed straight after each other with a rest or better known as a super set.
Once the superset has been completed, you can take a rest for a short break.
You can perform the superset 2- 4 times.
Exercise 1 : Hanging leg raises
Make sure when you are performing this exercise you pull your legs with your abdominal wall & not force your legs up using your hips. Try not to swing & use momentum as you will find your body will start to move around.
Exercise 2 : Swiss ball sit ups
Make sure you do not over extend on this movement by laying over the ball to much. Keep your hands either by the side of your head of placed on your chest along with your chin pointing away from your chest at all times.
Exercise 3: Russian Twists with a medicine ball
Make sure your knees are ever so slightly bent & raised off the floor by a few inches. Keep your upper body lent back slightly as this will create greater tension whilst performing the exercise.