Brad's Iyengar Yoga Notebook
When preparing for Dhanurasana, lie on your abdomen and first come up on your elbows and stretch your upper body and abdomen forward away from your pelvis on the mat to give you an extra inch or two in that direction.
Begin by practicing each leg individually, holding your shin bone, lifting your femur, focusing on not compressing your lower back.   You will probably find that practicing each leg individually is harder than doing the actual pose.
When you have conditioned each side first, then prepare to raise both legs together.   When first learning this asana, it is easier to do it with your knees separated somewhat, though the knees are held together in the classical form.   As you progress in the pose, first work on bringing your feet together, and then gradually work up to bringing your knees together.   Initially you can try to touch your great toes together and then progress from there.   Do not let your feet "sickle" inward though.   Extend through your inner legs and inner ankles and take your feet straight back and up.   This is also important in poses like Ustrasana, and in prone poses in general.
As you raise your torso and legs off the floor, lengthen your spine as much as possible in this pose.   Having the feeling of a smooth, round, long, and even arc from your head to your toes is more important than how high you can lift your torso and legs off the floor.   Seeking length before seeking height is one way you avoid too much spinal compression in the pose.   Elongate the front of your torso especially.   Move your chest more forward than upward.   Lead with your upper chest and collar bones, not with your Adam's apple.   This action is also important in Bhekasana and almost all backbends.   Strive to make your pose very long rather than very high.   This is also especially pertinent in an asana like Salabhasana.   Also try to achieve this feeling with your Dhanurasana.
Keep your lower abdomen on the mat, not your chest and not your pelvis.   It is your thigh bones that move upward in this pose.   Lift your thigh bones without pressing your pubic bone down on the floor.   Both your ribs and your thighs should be off the floor so that you are balancing on your lower abdomen.   Soften your abdomen, do not push the floor with it, so that your breath can be smooth and deep.   As a general rule, most students tend to focus more on the lifting of their chest than the lifting of their legs.   Try to balance those actions equally.
As with all backbends, your buttock muscles will tighten somewhat.   Roll the tops of your thighs inward, so as to keep your thighs and knees from turning outward with the tightening of your buttocks.   Backbend more from your hips than your lower back.
Broaden the balls of your feet and keep the balls of your feet pointing upward toward the ceiling (as in Sirsasana or Sarvangasana) rather than pointing your toes toward the wall behind you.   Lengthen your inner calves toward the ceiling.   As in almost all asanas, draw your shoulder blades into and down your back.   Take your dorsal spine in toward your chest to open your chest well.
Take care in all back bends not to compress the back of your neck.   The tendency we have when we are working hard in back bends is to allow the back of the neck to shorten or "scrunch up."   Remember to lengthen the back of your neck.   Relax your face, mouth, and eyes.