These planks are a classic core strengthener. My class loves it when I do these (I can tell from the sounds of deathly moans about halfway though these). You start on the floor in push up postion, only hold on your elbows and toes (wrap one fist in the other to help hold your form). Be sure to center your weight with arms perpendicular to floor (too far forward or back will tire your arms more than abs, or even hurt you). Suck your belly button in to your spine, and flatten that butt so I can set my glass of juice on it.
(this might be too hard to start off -if so, rest the first 2 sets on "C")