for thousands of years, yogis have noticed the profound effects of breathing practices, called pranayama. the goal of these practices is to soothe the body, calm the mind, and equalize the energies in the body. and the coolest thing is, you don't need a mat, any equipment, or even a teacher to tap into the benefits of the breath. all you need is a moment, and a willingness.
our thoughts and feelings are linked to our breath. ever notice when you get stressed or anxious your breathing gets more rapid and shallow? whereas when you're relaxed, it slows and deepens? well, you can also work this effect in reverse. if you slow and deepen your breath in a moment when your emotions are going haywire, you can begin to dissolve the tension instantly. very cool.
simple breathing exercise, such as "the perfect ten", where you attempt to count 10 consecutive exhales and inhales -- in and out through the nose -- without the mind wandering off (easier said than done), can be done just about anywhere at anytime. other pranayamas, such as alternate nostril breathing, are a bit more advanced, and can be used to balance the energies in the body, and also can effect which brain hemisphere is dominate and therefore, how we think.
so, when the going gets tough... breathe. just breathe.