Also known as Shirshasana, the head stand is one of the most important positions, or asanas, in hatha yoga, and is considered second in importance only to the lotus position, or padmasana. In fact, it's called "the king of asanas", possibly because of the combination of its relative ease (compared to the lotus position) and the benefits it is considered to bring. Some people beginning yoga practice are intimidated by the head stand, especially if they never did it when they were children, or if they think they don't have good enough balance, but it's actually far easier to get into than it looks, and doesn't take much strength. For most people, the only danger is of overbalancing and toppling over backwards, so at first it's good to practice in front of a wall.
How to do it
All you need are a yoga mat (if you're on a hard surface) and a towel to put your head on. It's not a good idea to use a pillow, because it's too soft and it interferes with your balance. Fold the towel up to about a foot square and place it in front of you, and kneel down, bending forwards until your forehead is resting on the towel. It's a good idea to stay in this position, which is know as the "child's position" for about thirty seconds, to let your body adjust to the extra blood flow to the head. This makes it less likely that you will feel faint when you go into the head stand. Let your arms relax by your sides and breath normally. *
After about thirty seconds, or just when you're ready, sit up a little and clasp your elbows with your hands, and lean forwards until your elbows are resting on the towel. Now let go of your elbows and clasp your hands. This ensures that your forearms form a triangle which is going to be the stable base of your head stand. Lean forward until the back of your head is cupped in your clasped hands, which should mean that the top of your head is on the towel. It may take some practice before you find the exact position in which to hold your head so that your balance is easy and your neck is not strained. It's important that the neck is not straining at an awkward angle to hold a head stand, but simply supporting your weight with the assistance of your arms.
Push yourself up with your legs, so that only your toes are resting on the ground, and walk yourself forward until most of your weight is supported on your head and your arms. When you feel comfortable, lift your feet off the ground and slowly straighten your legs until they are pointing straight up and your weight is balanced in one even line from your feet to the top of your head. It's important to do every part of this manoeuver slowly and under control. If you find yourself struggling or wobbling, stop, or let yourself relax back to the floor. Yoga isn't about straining to do the move the right way, it's about stretching and training your body gradually. Likewise, if you straighten your legs too quickly and find yourself overbalancing the other way, just let your body go limp and you'll drop to the floor without any problems. The main thing you want to avoid is injuring your neck, so take everything slowly and only do what you feel comfortably able to do.
Once you're up there, with your legs straight, don't forget to breathe! The temptation is to hold your breath and strain to keep your balance, but the more you relax, the easier it is. Breathe slowly, and aim to balance your weight more and more over your head, using your arms to take the main part of the strain. The more perfectly balanced you are, the less you have to tense and adjust your back and your legs to keep the position. It should be possible after a fair bit of practice to have most of your body extremely relaxed while in a head stand, which improves the blood flow and increases the benefit you'll get from it.
Keep the pose for as long as it feels comfortable. When you're starting off, about twenty or thirty seconds is probably all you will be able to do, but with practice you can stay in a head stand for a few minutes - three minutes is said to be the ideal length. When you feel like you want to come down, as with everything in yoga, do it slowly. Fast movements are dangerous in most forms of hatha yoga because they disrupt the energy and make it more likely that you will injure yourself. Bend your legs at the knees, then curve your back and bend at the waist to lower the rest of your legs until your toes touch the ground behind you gently. Then drop to your knees and take the weight off your head, and relax in the "child's position" again, letting your arms flop beside you. If you get up too quickly you can get a rush of blood away from the head that can make you faint and dizzy.
Why to do it
Head stands are good for you in several different ways. In Dharma Bums by Jack Kerouac, the protagonist, Ray Smith, is told a cure for his thrombosis by a hobo, who tells him to drink milk and honey and do a head stand every night. Head stands are also good for circulation problems, but their main benefits are to the brain and the spine. The increased blood flow to the head improves alertness, improves the quality of sleep and relieves mental stress. It is also said to help with headaches, eye problems, bad memory and fatigue, and even to promote hair growth. It is supposed to revitalize the endocrine system, stimulating the pineal, pituitary, thyroid and parathyroid glands. It is also good for back problems, because the act of balancing in a head stand places the spine in the correct alignment.
When not to do it
It's not a good idea to attempt a head stand if any of the following are true.
If you have a little bit of pain in your neck when doing a head stand, don't worry. Because you're supporting the main part of your weight on your arms, you will not injure your neck by compressing it in this way - in fact, it should be good for it. Just be careful not to try to do a head stand with your neck at an awkward angle, and try to do it in a place or time when you're not likely to be disturbed suddenly ("Mommy, mommy, look what Danny did to my knee!! Mommy? Mommy, are you ok? Why were you upside down in front of the door? Mommy, should I call the am-bu-lance?").
It's a good idea to at least read a book or look at pictures of the correct posture for a head stand, but by far the best way to learn any asana is to be taught how to do it by an instructor. It might save you an injury or a long-term strain, as happened to me when I pushed myself too hard to get into the lotus position without having flexible enough hips, and ended up straining the ligaments in my knees. You can find some pictures in the URLs below (and even an animated gif showing how to get into the pose). Individual instructions on how exactly to do any asana may differ, because everyone's body is slightly different, and everyone develops their own way of doing yoga which suits their needs best. The main thing to remember is to do what feels good, and not to do what feels bad.
* Note - "normally" in hatha yoga means that you are breathing deeply and slowly, from your belly, and often making a rasping noise known as
ujjayi in your throat.
Yogic breathing deserves its own writeup but the main thing is, slow, controlled, deep breaths that come from the pit of the stomach rather than the chest.
References, pictures and further reading:
http://www.santosha.com/asanas/sirsha.html
http://www.holistic-online.com/Yoga/hol_yoga_pos_headstand.htm
http://www.duke.edu/~das19/yoga/headstand.html
http://www.jasongriffin.net/asanas/headstand.htm