Chaturanga Dandasana, the four footed staff pose is a challenging posture for both beginning and intermediate practitioners to master. In the posture, the whole body is parallel to the floor, stiff as a staff and straight as an arrow, with the four limbs equally supporting the body.
This posture requires complete body integration. From the hands to the feet, head to the heels, front to the back, and even the sides of the body.
Common mistakes or misalignment in the pose include anterior rotation (rolling forward) in the shoulders, collapsing in the chest, arching in the lower back, and disengagement in the legs.
Many beginning students make the error of attempting the full posture too soon without taking the time to practice the strength building components in the foundational work. These modifications which Anne Phyfe mentioned, such as knees to the floor, build strength in the arms, chest and back. Reducing the common "Chaturanga collapse."
To refine the art of this posture and reduce some common problems in the pose, try practicing Chaturanga Dandasana with a strap. This Iyengar technique was taught to me many years ago by instructor Jo Leffingwell:

1.) Take a yoga belt (preferably with a metal buckle) and make a loop approximately 2 inches wider than the distance of both shoulders. To measure, stretch the loop across your chest. Each end of your belt should touch the mid-portion of each upper arm.


2.) Sit in Vajrasana.
3.) Thread both arms through the belt placing it just above the inner elbows. Make sure to keep the buckle side of the strap away from your skin. Bend the elbows along your sides as you would for Chaturanga Dandasana and draw the belt against your lower ribs and upper waist.

4.) Spread the fingers,place the hands on the floor and rest the trunk of your body on the belt as if it were resting on a sling or hammock. Do not allow the breast tissue to come in contact with the belt.

5.) Step one leg back at a time, feet about a foot apart and enter this supported variation of Chaturanga Dandasana. You should feel very supported by the belt.

6.) Now begin to work some of the key actions of the pose:
*Activate the whole body, parallel to the floor from head to heels.
*Compact the shoulder blades against the back and keep the upper arms parallel to the floor. Make sure that the shoulders do not roll inward towards the floor or fall way below the elbows.
*Squeeze the elbows in toward the mid-line and hug the muscles along the side-body closer towards the bones. Keep the elbows along your sides. Do not allow them to support the front ribs.
*Draw the pit of the abdomen in, lengthen the tailbone down and lift through both Uddiyana and Mula Bandhas. Try not to collapse in the trunk but keep it strong, elongated and parallel to the floor.
*Keep the legs strong and active by lifting the quadraceps and kneecaps upwards. Make your legs equally as potent as the arms.
8.) Rest and try again.
Try not to get discouraged. Chaturanga comes with regular practice. Ask your teacher to assess your form and alignment in Chaturanga in order to support your work in the refining of this pose further. Chaturanga Dandasana should be tried gradually by beginners in order to develop significant mobility and power.