The 4-7-8 Breathing Exercise is utterly simple, takes almost no time, requires no equipment and can be done anywhere. Although you can do the exercise in any position, sit with your back straight while learning the exercise. Place the tip of your tongue against the ridge of tissue just behind your upper front teeth, and keep it there through the entire exercise. You will be exhaling through your mouth around your tongue; try pursing your lips slightly if this seems awkward.
1) Exhale completely through your mouth, making a whoosh sound.
2) Close your mouth and inhale quietly through your nose to a mental count of four.
3) Hold your breath for a count of seven.
4) Exhale completely through your mouth, making a whoosh sound to a count of eight.
5) This is one breath. Now inhale again and repeat the cycle three more times for a total of four breaths.
Note that you always inhale quietly through your nose and exhale audibly through your mouth. The tip of your tongue stays in position the whole time. Exhalation takes twice as long as inhalation. The absolute time you spend on each phase is not important; the ratio of 4:7:8 is important. If you have trouble holding your breath, speed the exercise up but keep to the ratio of 4:7:8 for the three phases. With practice you can slow it all down and get used to inhaling and exhaling more and more deeply.
More info: drweil.com