What makes performing planks so rich, despite their lack of movement (they are primarily an isometric hold) is that they allow you to work deeply on your postural alignment. You must quiet your mind and pay close attention to engaging your core muscles, so that they are working together to maintain an upright posture without getting tense.
It is important to make sure that your hips and shoulders remain properly stacked over each other. If you allow one or the other to twist out of alignment, you will end up allowing part of your oblique muscles to dominate. By practicing any movement incorrectly, you are setting yourself up for potential injury and pain.
Before getting into this position, think of how to stand correctly. You must align your ears over you shoulders, your shoulders over your hips and your hips over your ankles. Many of us tend to jut our heads out in front of our shoulders and allow our shoulders to round forward.
To correct this, think of:
1. Lengthening the two sides of your body, from your hips to your underarms.
2. Drawing the base of your shoulder blades toward the center of your spine (Anusara yoga refers to the spot as your heart center) and the tops of your shoulder blades away from each other.
3. Your collarbones lengthening out to the sides.
4. Lengthening the two sides of your neck.
5. Keeping the tip of your tongue gently pressed against your upper palate.
6. Drawing your navel toward your spine without rounding your back.
With all of that in mind, lean on your forearm, lining your elbow up with your hip. Push away from the floor so that you are not sinking into your waist or shoulder and lift your hips off the floor while keeping your feet stacked one on top of the other. Do not allow yourself to lean forward or back. Engage your inner thighs by hugging your legs together. Hold this position for 15 to 60 seconds.
Keep your knees bent and on the floor. This shortens the lever (your body) and thus makes the exercise easier.