This article comes from the Winter 2010 issue of Good magazine (issue #018):

The Sun and Moon Breath is an ancient technique that balances you right out. The moon breath (left nostril) calms your nervous system while the sun breath (right nostril) perks you up. It totally works. For the purposes of this guide, try the Moon Breath.  Really. Try it. Now. Right now.

1.  Place your right thumb over your right nostril.
2.  Close your mouth, and breathe in very deeply and very slowly 30 times through your left nostril.
3.  This will cause a physiological change in your body and brain – it will mellow you out.