The plank is an isometric (no range of motion - like clenching your fist and holding that squeeze) core exercise in which you're basically holding a challenging position for an extended amount of time. The beauty is that your strengthening your abs and your back simultaneously.
The basic plank goes like this: Get on the floor face down like the top part of a push-up. Next, bend your arms and rest your weight on your elbows rather than your hands. You're on your toes and elbows, and your body should be a straight line, with your head in a neutral position. Tighten your stomach as if you're about to be punched, and don't forget to BREATHE. Hold this position as long as you can. I have my clients work up to holding this basic plank for a full minute before playing with other fun(?) varieties.
As you hold the plank, it's natural for your hips to start sinking - don't let them! Remain straight. Your body may try to cheat by sticking your hips up too high, making the move easier. Remember - it's a plank...as in a plank of wood. Straight as a board. (Unlike the cheap warped wood you may find at your local home improvement store.)
If you can't hold yourself up as described for several seconds at a clip, you can regress the plank by increasing the angle between your body and the ground. For example, try planking with you hands (instead of elbows) on the side of your couch, or on a low countertop. Everything else applies - be straight as an arrow, stay tight and regulate breathing. You can use this same regression for push-up's too.
I always enjoy watching clients' planking ability improve - and it does improve in fairly short order. Try to do a little longer each workout, and you'll be a 9th level Zen plank master in no time.
The payoff? Proper planking can assist with back pain, improve posture, reduce the risk of back injury by strengthening the supporting cast, and improve those abs. Tough to beat.
Train smart and be well!