The amazing thing about plank, and all it's variations, is that although a majority of work is placed on the abdominals, the entire core, including the back, are engaged. Not to mention your quads, shoulders, even your pecs.
Actively focusing on pulling the belly button in, the plank is one of those core exercises that will actually shrink your waist line. All too often I see people doing ab training with moves that will actually thicken their waists. Of course as always, I believe in making sure you have the basics first. Work on getting your time in plank up to 2 minutes, then 3, and then before you know it, 5 minutes will be a piece of cake.
If you're looking for something a little more challenging though, try the "Two Point Plank" shown below. Starting in a high plank on your hands instead of your elbow, alternate raising your opposite arm and leg for a long stretch. Carefully switch sides, for 10-20 reps each side. The trick to balancing in any move where you are stabilizing, is about LENGTH. When you reach forward with your arm, actually reach further. Don't just dangle your hand. Same for the leg, don't just casually lift... lift UP and OUT, and lengthen your entire body. Not only will this help engage even more of your core, but it will help you stabilize as you move side to side.
Coming up - I will be posting a video of my favorite plank variations. You won't want to miss the demo.