Few things hurt as much as lower back pain, and Headstand must be done and especially exited properly in order to avoid that particular unpleasantness. In fact, before making any technical recommendations, we should mention that Headstand should not be attempted by yogic "weekend warriors," and is best learned under the watchful eye (and spotting arms) of an experienced teacher.
The key to coming out of Headstand correctly is keeping the front of the torso long as you begin to lower the legs, says YJ contributing editor and longtime Iyengar instructor Richard Rosen, who recommends the following method: First, exhale and bend your knees, dropping them toward the floor. You can bring your thighs in toward your abdomen and separate your knees about hip-width, so you're in a kind of upside-down squat. Don't curl your spine; keep your tailbone pointing straight toward the ceiling. Hang out in this "squat" for a bit. Then, draw your energy along a line running from the inner knees to the inner groins and deep into the pelvis. "This helps support the position and protect the lower back," says Rosen.
From the bent-knee position, inhale and extend the knees so the straightening legs angle down somewhat toward the floor. Two cautions: If the hamstrings are tight, the legs may not straighten fully at first. Also, if the arms and back are weak, the feet may come crashing down to the floor, so be careful here.
To help support this part of the descent, press the shoulder blades into the back of the torso as if doing an upper-back bend, and then pull them down strongly toward the tailbone. Remember to keep the front torso long! Also, be sure not to drop the pelvis back too far to counterbalance the legs, since that can also upset the back.
As with the shoulder blades, press your sacrum into the back; then exhale slowly and, as lightly as possible, lower the feet to the floor. Ideally, this last movement should take 10 to 15 seconds. Finally, end by bending the knees and nestle into Child's Pose (Balasana).