The key to a good Ustrasana, in my experience, is keeping the legs active all the way down to the toes. Pressing the tops of the feet firmly into the mat provides solid grounding from which to bring the hips forward over the knees and distribute the bend evenly along the length of the spine, rather than hinging from the lower back. Iyengar recommends achieving this action by "pushing the spine into the thighs, which should be kept perpendicular to the floor (Light on Yoga)." Honestly, I'm not sure exactly how to push my spine into my thighs, but it sounds like it hurts. I think he's telling us to tuck the tailbone and maintain length in the lumbar and sacral spine. Leslie Kaminoff suggests in Yoga Anatomy that "a mild internal rotation of the legs is recommended to keep the sacroiliac joint stable," which also aids in bringing the hips forward and opens the front of the hips for a good stretch to the hip flexors.
I like to come into this pose gradually, taking a few breaths with the hands on the sacrum, breathing deeply into the expanded chest and slowly curling the spine back one vertebrae at a time before dropping the hands to the heels. I generally warm up the back body and shoulders with a series of Shalabasana variations and a Dhanurasana or two, as per the Ashtanga 2nd series, but I find that Ustrasana is also especially nice as a counter pose to arm balancing poses, such as Bakasana, or any targeted abdominal work.
As with most of the big heart and throat openers, Ustrasana is not without its challenges. Luckily, modifications abound for this pose. One may modify by keeping the hands on the sacrum, pressing the hips forward with the hands and maybe stretching the head back, or by curling the toes under to bring the heels closer to the hands and assist in the action of bringing the hips forward over the knees. One modification I particularly enjoy is bringing the thumbs into the armpits, which, though kind of gross if you're really sweaty, is a very effective position for opening the chest and bringing the shoulders down and back in preparation for the full pose.
When first learning Ustrasana, the hardest part for me was breathing. My chest, shoulders, and throat were all very tense, and getting any air through that squished trachea was difficult. As Kaminoff puts it:
"The thoracic structures are maintained in an 'inhaled' position.... This results in a decreased ability to breath 'normally.' The trick is to find support from the deeper musculature so the more superficial efforts can quiet down. Then it's possible to notice an interesting relationship between the deepest layer of superficial neck muscles (scalenes) and the breath movement in the apex of the lungs, which are suspended from the inner scalene muscles (Yoga Anatomy)."Hmm... interesting relationships, eh? Fascinating stuff. But it's true, breathing well and maintaining a healthy position of the head and neck when practicing Ustrasana can be the trickiest part. I know heart and throat opening asanas can be intense for many practitioners. What's your experience with Ustrasana?