Although not technically a yoga pose, practicing the plank is really a great way of improving your yoga practice. The plank is a pilates move and really focuses on strengthening the body's core. It can be incredibly difficult at first and beginners are encouraged to try it in short sets, taking care not to strain their bodies.
To begin with, you will start on your hands and knees, with your arms outstretched in front of you, in a leaping frog sort of position. In a swift movement, you will straighten your legs behind you, lift your body off of the ground, and let your arms support you from underneath. At this point your hands should be directly underneath your shoulders and your arms should be taut. Your feet should rest entirely on the floor and you should resist the urge to lift onto your toes. Your body should be straight and as parallel to the floor as possible.
The most common mistake when doing the plank is the tendency to let the core drag -- to let the belly go soft or to the let the lower back sway. If right away you notice this and cannot correct it easily, it is perhaps because your core is not strong enough yet -- you should try the modified plank as will be described below. Letting the belly drop and the back sway or curve is very dangerous, so be sure to not let this happen.
The modified plank is much like the full plank but is done with the knees on the floor (similar to a modified push up). With your knees on the floor, you will push yourself up again so that your body is as straight as possible and your core muscles are engaged. While it might not feel challenging at first, after you hold this position for about 30-40 seconds you should feel the burn. Those in the full plank should release out of the position after about 30 seconds.
Wearing a watch while practicing yoga is important so that you can watch the time as you perform your poses. Straining to get a glimpse of the clock is not wise given the difficulty of many of the positions.