The plank, as I'm demoing above, is one of those really great basic exercises that handles a few things at once. It's core strengthening as you're contracting and holding in those abs while holding your body in as straight a line as you can. I love doing these and do them just about every day. I do 30 second holds. If you're just starting out, begin at 20 second holds and slowly build up to longer times. In my classes on days when I'm being especially brutal, I make my class members hold these for up to a minute! ;)
Variations on the plank are many. You can do what I call the 'yoga plank', which is just the same move pictured, but up on your hands instead of your elbows, arms nice and straight. You can also do one legged holds, alternating knee ins (draw knee up and in towards the center of your chest, one at a time), and bi-level planks, where you switch back and forth from the elbow plank to the yoga plank over and over again. Those are killer, and also great for strengthening your triceps if you ever decide to try them.
Ok! So, off to do my own morning routine before I head out the door to teach my classes! If the plank is not a part of your present exercise routine, get it in there! These are great, and you will begin to see the results of these sooner than later. Go, now and do it! :)