1. Put your right hand up to your nose. Hold your index and middle fingers on your forehead to hold your hand stable. You'll notice your thumb is on the right side of your nose and your ring and little fingers are on the left side.
2. Now use your thumb to plug your right nostril. Take a slow, deep breath in through your left nostril, counting to eight. Slow down your in-breath so it takes eight seconds to fill your lungs.
3. Plug your left nostril (so both sides are now blocked) and hold your breath to a count of eight.
4. Now lift your thumb off your right nostril (keeping your left nostril plugged) and breathe out steadily, through your right nostril only, for a count of eight.
5. Do not pause at the end of the breath. Immediately start breathing in and breathe in through the right nostril to a count of eight.
6. Plug both sides and hold your breathe for a count of eight.
7. Now breathe out through your left nostril for a count of eight.
8. Start all over again, breathing in through your left nostril.
Breathe in and out as quietly as you can. This makes your breath slow and even.
This seems a lot more complicated than it is. It's very simple once you've done it a couple times.
This technique occupies your mind. All the holding and counting is absorbing. This simple activity successfully keeps out other thoughts, allowing you to get lost in it. It is easier to concentrate on alternate nostril breathing than on a mantra. And it is very relaxing.
It is a scientific fact that your nostrils normally change dominance. Throughout the day, without using any technique, the blood flow alternates every couple hours between the left and right sides of the nose, causing first one and then the other nostril to become more congested, allowing air to flow more easily into and out of the uncongested nostril.
Apparently this shift back and forth every 90 to 120 minutes is associated with brain hemisphere dominance. When the left nostril is more open, people test better on right hemisphere tasks like spatial relations. When the right nostril is more open, people do better at left-brain tasks like verbal expression.
I'm speculating now, but it's possible that alternate breathing balances the activity of the two hemispheres of your brain so that neither is dominating the other. What ultimate difference this makes, I don't know, but it sure feels good and is very relaxing. Doing it for a few minutes is a great preparation for meditation too. To make it easier to do this exercise, here is the technique in condensed form:
IN THROUGH THE LEFT
OUT THROUGH THE RIGHT
IN THROUGH THE RIGHT
OUT THROUGH THE LEFT
IN THROUGH THE LEFT
Each in, out, and hold is done to a count of eight (approximately one second per count).
You can do a less complex version of this. Just plug your nose on one side, breath out, breath in, switch plugs, breathe out, breathe in, etc. Count to three on each side, or four or five. That is from the excellent book, Conscious Breathing: Breathwork for Health, Stress Release, and Personal Mastery.