An effective weight training regimen can be greatly enhanced not only by including regular stretching as part of the routine, but also by maintaining proper focus on breathing. Like stretching, breathing is often overlooked as a skill which practiced regularly will enhance a workout routine.
Regular focus on your breathing, even when you're not working out, can help to restore energy, reduce stress, improve concentration, and otherwise supply a more robust source of energy for the body.
Perhaps the most important principle to keep in mind is that one should never hold their breath during exercise. Not only does this increased stress, but it deprives the body of energy necessary to complete the exercise. The next most important principle to observe is that breathing should be rhythmic and controlled. Many people who exercise find themselves out of breath ("hyperventilating"), because they have allowed their breathing to the uncontrolled. It is possible however to focus your attention on your breathing in a way that avoids being out of breath, even during intense exercise.
In the Western world, we have a tendency to breathing from our chest, which typically into being very shallow. The result is that our bodies receive much less energy flow. Breathing from the chest also occurs and has a tendency to promote stress. Asian cultures, especially the Chinese, have typically focused on breathing from the lower abdomen, more specifically an area 1 1/2 inches below the navel. Try this the next time you have a few moments. A better description is as follows:
"Inhale the way that the area just below your navel contracts as you taking the air. Breathing out such that the same area expands as you exhale. Do this in a slow and rhythmic fashion until you can consistently inhale while you're abdomen presses in, and exhale as your abdomen expands."
The Chinese have focused significant effort into the study of breathing as it relates to health and martial arts. In the process, a systematic approach to breathing has emerged, and has become a major focus of many martial arts.
Breathing can also be the basis of a meditation session to start and/or end the day. When done for this purpose, the primary goal is stress reduction and conditioning the body to control the process of breathing.