The plank helps develop overall core strength and stability. Hold yourself up using both elbows in the prone position and hold the position for as long as you possibly can.
Also, incorporate the side plank into your ab routine, which is basically the same as the normal plank, only you lean on one elbow at the time and switch sides to hit both your left and right obliques. Once you've grown accustomed to the exercise, grab a dumbbell to do loaded side planks.
Bring the dumbbell to your hip and then push yourself, along with the dumbbell, up into starting position. Balance the dumbbell on your top hip.
Hold for as long as possible and then switch to the other side. Rinse and repeat.