Brad's Iyengar Yoga Notebook
Like Adho Mukha Vrksasana, this pose involves learning to open your shoulders, and similarly, you use a wall to assist with balance until the pose becomes easy.   Classically this pose is essentially a vertical one.   Although there will be some arch in your back naturally, you aren't specifically trying to achieve any sort of real backbend such as in Vrschikasana.
You may find it helpful to use a belt around your elbows to keep them from splaying outward and to keep your forearms parallel.   Similarly, having your hands on either side of a block placed against the wall will keep your hands from moving inward and provide extra leverage for the pose.
Actions of the hands, arms, and shoulders
Before coming into the pose, place your forearms on the floor parallel to each other (perpendicular to the wall).   First turn your palms up on either side of your block (if you are using one) and tuck your elbows and forearm skin underneath medially and then keep that position in your elbows and forearm skin as you turn your palms only back downward beside your block.   Spread your hands as in Adho Mukha Svanasana (although not quite so far apart) and keep your middle fingers parallel (or spread your fingers even less if you are using a block).   Press down on the floor with your hands and fingers more than with your elbows while you are in the pose.   Press your inner wrists down to the mat and press your forearms down so the backs of your wrists are level.   If you pour water on the back of your wrist, would it run off to the outside?   Try to minimize the litte "cave" underneath your inner wrist bone.   Also, be on your inner elbows.
Your shoulders must remain out directly over your elbows before coming into the pose and then throughout the pose.   Keeping your shoulders out over your elbows helps you to maintain the openness of your shoulders and chest.   Pull your shoulders strongly away from the wall to open your axillae (armpits) maximally.   Open your axillae and shoulders so your upper arms do not slant back toward the wall -- they should be vertical, perpendicular to the floor.   Open your axillae more and more toward the center of the room which is also opening your shoulders.
Lift your shoulders and raise your shoulder blades strongly toward the ceiling.   Keep your upper arms rotating outward to assist in broadening your collarbones outward to the sides.
As you come up into the pose, do your best to step up gracefully into the pose rather than just throwing your feet up to the wall and expecting the wall to stop your momentum.   Try to come up into the balance without even touching the wall if you can.   As you come up into the pose, do not allow your arms to shake or oscillate.   Just as with your legs when coming into the standing poses, you want to be strong in your arms here and not allow them to lose the stability you have established in them as you come into the pose.
Actions of the legs and feet
Keep your legs active by squeezing your thighs, knees, lower legs, and ankles together.   Squeeze your muscles into your legs and up toward the ceiling.   Reach upward through the insides of your feet and legs more than the outsides.   (The front sides and back of your thighs should move upward equally, but you will likely have to work harder at lifting the inner thighs to achieve this, as in Sirsasana, Sarvangasana, and most inversions).   Turn the front of your thighs inward to keep them straight ahead, since they have a tendency to roll outward.
Observe the Tadasana of the legs and give them some focus upward toward the ceiling.   Broaden the soles of your feet and the balls of your feet and spread your toes.   Extend both through your heels and also through the balls of your feet (especially at the great toes).   Keep your legs lifting strongly out of your pelvis.   Raise your hips toward the ceiling to elongated your torso.
Try to balance without touching the wall, then try to learn to come into the pose without having to touch the wall first.   If you are learning the balancing, if you touch the wall coming up, don't accept it -- come down and go back up again.   Learn not to use the wall at all when you come up.  It is easier to balance without the wall in this pose than in Adho Mukha Vrksasana (full arm balance), but harder than in Sirsasana.   If you are able to balance without the use of a wall, your heels should be directly in line with the top of your head.
Actions of the torso, hips, and pelvis
Minimize the arch in your low back, though it will always be there somewhat.   Draw your navel toward the wall behind you by using your spine, not your abdominal muscles.   Draw your floating ribs back toward the wall.   Try to create the backbending flavor more in your upper back and shoulder area rather than in your low back so much.   Lift your ribcage upward toward the ceiling.   Lift your lower ribs away from the floor.   Lift up out of your armpits.   Have your torso over your arms, not sagging out in front (away from the wall) much.   This position comes from opening your shoulders.   If you are using a wall, try to minimize its use.
Lift your pelvis upward, taking its weight off your torso, and allowing you to lengthen your spine maximally.   Lift your tailbone toward your heels.   Lengthen the sides of your trunk and lift them upwards toward the ceiling.
Broaden your collarbones from side to side.
In the classical pose, you turn your head to look at the floor between your hands or at your thumbs.   You can also let your head hang so you look toward the center of the room.
Learn to come into Pinca Mayurasana out of Sirsasana.   Strive to come out of the pose gently with straight legs, preferably both together, though this is a more advanced action.