Brad's Iyengar Yoga Notebook
Adho Mukha Vrksasana
Adho Mukha Vrksasana (full arm balance) is a good asana to learn to open your shoulders and to develop arm and wrist strength.   The preparatory poses for full arm balance are Adho Mukha Svanasana (downward facing dog pose) and plank pose.   You should develop stability and arm strength in these asanas before attempting full arm balance.
You should practice with your back to a wall until you develop the ability to balance without the wall.   Even when you are using a wall, try to minimize its use.   Try to learn to swing your feet up to touch the wall as softly and silently as possible.   Initially, it will be easier to swing one leg up first with the other following behind it.   Coming up with both legs together is the more advanced way of entering the pose.
To swing your legs up successively, first place your hands and walk in toward your hands with your feet as much as possible before kicking up.   Have your hands about one hand-length away from the wall to begin with.   Having your hands closer to the wall makes it more difficult to swing up into the pose.   Lift your hips as high as possible before bouncing up into the pose.   The higher you get your hips, the easier it is to get up.   Take one leg straight up behind you as far as possible, bend the other one, and use your lower leg to bounce you up.   When you have raised your upper leg as high as you can behind you, try not to drop it again to gain momentum to bounce up.   Your upper leg pulls you up as your lower leg pushes you up.   The more height you can get in the upper leg to begin with, the less the lower leg will have to bounce you up.   Remember to change legs each time you go up to develop both sides evenly and avoid repetitive stress injury.   Do these same leg actions also to get up into Pinca Mayurasana.
If you are unable to swing your legs up into the full pose yet, you can still work on the pose by practicing Ardha Adho Mukha Vrksasana by placing your hands on the floor one leg-length away from the wall, heels of your palms toward the wall, and walking up the wall with your feet until your hips are at a 90 degree angle and you are doing full arm balance in your upper body.   Similarly, you can practice in a doorway, with your back to one side of the doorway, walking up the other side of the doorway with your feet.   Then it is easier to swing your legs up into the full pose from this shorter distance than it is to come up all the way from the floor.
Spend some practice sessions just bouncing up into full arm balance and then coming right back down and repeating.   One good way of working in this pose is to kick up with your left leg and your right leg follows and then drop your left leg and your right leg follows to the floor.   Then repeat that action 10 times.   Then switch with your right leg coming up first and coming down first not holding the pose very long just working on entering the pose from both sides.   The more you practice like this, the more gracefully you will be able to come up into the pose.   Try to learn to step up gently and gracefully into the pose rather than throwing yourself toward the wall.
Coming up with both legs at the same time at the wall is a more difficult maneuver.   You have to bounce them up together with bent knees and then straighten them up the wall.   Much more difficult still, after you are able to balance in the center of the room, is to come up with straight legs as in the classic way to enter Sirsasana.
Using a belt around your arms about 1/2 an inch above your elbows may make it a little more difficult to move up into the posture (though for some people it is easier), but it will allow you to double your time in the pose once you are up by reinforcing your arm position.
If you use a wall, be close enough to the wall to prevent backbending.   A common mistake is to be too far away from the wall and to rest the heels on the wall, arching backward and hyperextending the spine.
Once you are in the pose, try to get a feeling for the balance by taking your feet away from the wall.   One way to do this is by moving one foot away from the wall and extending upward maximally with it, then the other foot, and eventually taking both feet together away from the wall.   Perhaps more properly, however, you should take both your feet away from the wall together.   When working on balance, one trick is to come a little closer to the wall than normal and to place the top of your head on the wall for balance, bringing your feet and everything else away from the wall, using your head on the wall for balance.   Then slowly lower your head away from the wall when you're ready.
If you practice full arm balance with your hands on blocks turned on their sides (gripping the blocks), it will take some of the weight out of your wrists, though it will be a little more difficult at first to raise your feet up.   Have the blocks touching the wall for support.   Practicing the pose on blocks creates a psychological feeling of lightness.
Most often this pose is done with the crown of the head toward the floor, ears in line with the arms, though in the classical pose, you turn your head and gaze toward the floor.   If you chose the latter method, take care not to compress the back of your neck
Actions of the hands, arms, and shoulders
Using the wall as a reference, this pose can be done with your fingers directly facing the wall, with your hands turned outward (90 degrees, for instance), or even with your fingers pointing 180 degrees directly away from the wall (with the volar aspect of your forearms facing the wall).   The point of turning your hands in different directions is to work on mobility in your upper arms, shoulders, and neck region.   Also, turning the hands outward somewhat, up to 90 degrees, helps some people maintain the straightness of their arms.   Most people find the pose easiest when turning the hands outward at least partially, next most difficult with the fingers facing directly to the wall, and the hardest with the fingers facing 180 degrees away from the wall.   Whichever direction you choose for your fingers, you want to guage the distance between your arms by what distance will keep them each vertical, perpendicular to the floor, not just by the space between your hands.
Stretch your hands on the floor as in Adho Mukha Svanasana, stretching your fingers forward with your middle fingers parallel if your fingers are facing the wall.   Bring your weight into your metacarpals, not so much into your carpal or wrist bones.   Just like in Adho Mukha Svanasana, you want most of your weight to be born by your palms and metacarpal bones and not by your wrists.
Maintain your elbows and shoulders directly over your hands.   Lift the skin of your forearms away from the floor.   Lift your shoulders up strongly away from your wrists by contracting your inner deltoid muscles.   Keep raising your collar bones higher and higher to their maximum height throughout the pose.   Stretch your outer armpits strongly.   Open your armpits maximally and feel that they become thinner and thinner as you stretch upward.   Raise your trapezius muscles.   Feel your outer shoulderblades move downward in the direction of your arms while your inner shoulderblades lift upward toward your feet.
Actions of the torso, hips, and pelvis
Just before you swing your legs up into the pose, bring your pelvis directly over your shoulders and arms and keep it there during the pose.   Keep your hips directly over your shoulders.   Lengthen your body maximally upward from your hands up through your heels and the balls of your feet.   Lift and lengthen the sides of your trunk.
Lift your pelvis upward, taking its weight off your torso, and allowing you to lengthen your spine as much as possible.   Tuck your tailbone in to counteract your thigh bones moving backward, but do not push your groins forward.   Minimize the arch in your low back.
Actions of the legs and feet
When upside-down, there is the tendency to lose the consciousness of the legs since they are no longer supporting the weight of the body.   You should pay attention to lifting your legs out of your pelvis toward the ceiling and engaging your leg muscles to the bones just as consciously as in the standing asanas.   Squeeze your muscles into your legs and up toward the ceiling, lengthening your legs through the balls of your feet.   Keep your legs active by squeezing your thighs, knees, lower legs, and ankles together.   Take your thigh bones deep into the backs of your legs just as in the standing poses.
Turn your thighs inward slightly so they face front as in Tadasana.   Extend your heels toward the ceiling maximally.   Slide your heels up the wall more and more so that your back body lifts higher and higher.   Raise your heels to raise your hips.   Reach upward through the insides of your feet and legs a little more than the outsides.
Come out of the pose by coming down with both legs straight if possible and landing gently in Uttanasana.   If you cannot come down gently with both legs, come down by lowering one leg as much as possible while keeping the other one up in the pose, landing as softly as you can with each leg successively.   Remember to alternate which leg you come down with each time you do the pose.   Make it a general rule to do Uttanasana after this pose.
One to two minutes is a good time to aim for in this pose.   Guruji Iyengar can do five minutes even past eighty years old.
If you have significant wrist discomfort after the pose, have a partner grasp your forearm and hand and pull them apart strongly, as if to separate them at the wrist and repeat that on the other side.