While this particular technique can be practiced anytime, it's particularly effective when you're focused and relaxed. To begin, inhale through your nose for at least two seconds, feeling the air move into your abdomen. Try to fill your abdomen with air instead of just your lungs. Purse your lips like you're blowing on hot food or whistling and exhale slowly for four seconds. Repeat for 3 to 4 breaths. Remember to stop immediately if you begin to feel light-headed or dizzy.
While this technique can be practiced while standing or sitting, laying down seems to help most people do it correctly. Place one hand on your chest and one hand on your belly. Breathe in through your nose for about two seconds, feeling your stomach expand. The hand on your chest should barely move at all. Purse your lips (as if you're about to drink through a straw), press gently on your stomach, and exhale slowly for about two seconds. Again, the hand on your chest should barely move. Repeat.