Breathe! Breath is life. We can live for days without food or water, but deprive one of breath and we die in minutes.
Most people have forgotten how to breathe properly. They breathe shallowly, through the mouth, and make little or no use of the diaphragm.
Breathing correctly means breathing through the nose, keeping the mouth closed, and involves a full inhalation and exhalation, which bring the whole of your lungs into play.
There are three basic types of breathing – clavicular (collarbone/shallow), Intercostal (ribcage/middle) and abdominal (belly/deep). A full breath combines all three, beginning with a deep breath and continuing the inhalation through the intercostals and clavicular areas.
There are three parts to each breath – inhalation, retention, and exhalation. Exhalation is actually the key to breathing, for the more stale air you exhale, the more fresh air you can inhale.
Begin by breathing into the belly, as if you are imagining blowing up a balloon in your belly by breathing in. As your belly is full of air, then continue to fill up the middle of your lungs, all the way to the top. Hold the breath for a short while, and exhale, in the opposite order. The exhalation should take about twice as long as the inhalation. As you near the end of your exhalation, your belly, abdominals, and diaghram should contract, squeezing the last bit of air out.
The above basic breathing is a form from Yoga. There are actually breathing routines to help improve life, vitality, and well being. The best way to learn more about breathing and the various routines is to attend Yoga classes, or you can check out a good book on Yoga. The Sivananda Companion to Yoga by the sivananda yoga center is one good one I know of.