Abs Circuit 3

Swiss Ball jack-knives  (15-20 reps)
Side crunches  (10 reps each side)
Plank (1 minute)
Leg raises  (20 reps)
Knee up crunches  (30-40 reps)
Russian twists  (1 minute)

Unsure of a few moves? Jackknives can be found in section 2011, august, day 6. Side crunches are perfromed from lying on your side with the hand underneath placed across your chest holding the opposite shoulder. The other hand is in the normal sit up position. Now raise your legs and crunch your body upwards into a slight v shape and lower back down again.
Russian twists are performed from a sit up position with your feet off the ground, either out straight or knees bent. Then keeping both hands together, twist your body to each side and touch both hands to the floor, then twist the opposite way and repeat.

Timing and duration:
For beginners try completing the circuit twice with a rest time in between each circuit of 2-3 minutes with as little rest as possible between each exercise.
For a more advanced workout try completing the circuit 3 times with a rest between each circuit of 1-2 minutes and no rest between exercises.

Want to go even harder, then add weight to the Russian twists in the form of a small dumbbell 2-5kg should be fine. Or hold the plank for longer, up to 2 minutes or beyond. Or simply do more reps of the crunches, leg raises or jack-knives.

But most importantly finish the circuit in a controlled but high effort way for best results. Remember you are only cheating yourself if you cheat the circuit.