Planks are one of the best exercises to strengthen the core. Try this workout to really challenge the core. Rest for 30 sec after each exercise. Do twice a week and be on your way to fantastic abs!
Reptile plank- 4 sets of 20 (10 on each side)
Plank on the forearms for 1 min
Plank with knee to opposite elbow- 4 sets of 20 (10 on each side)
Plank for 1 min on forearms
Twisting plank- 4 sets of 20- 10 on each side
Side Planks- 30 sec on each side - twice