Relaxed Diaphragmatic Breathing
When feeling stressed, restless, or on edge, it can help to learn the technique of correct breathing. Many people breathe through their chest. Shallow breathing through the chest means you are disrupting the balance of oxygen and carbon dioxide necessary to be in a relaxed state.
Do this exercise twice a day, once at night before you go to bed and once in the morning right after you awaken. At these times you should be wearing loose fitting clothes and be able to easily lie down.
Lie down, place one hand on your abdomen over your navel, and breathe in such a way that it moves up and down in a relaxed manner. Your chest may move a little while you do this; however, most of the movement needs to be in the abdominal region.
Each practice session needs to last only about a minute, just long enough to take four or five relaxed and comfortable breaths. Do not make this hard work. It is also important to avoid over breathing. The type of breathing you are practicing is a very gentle and relaxed form of breathing. If you breathe too deeply or too rapidly instead of taking slow, gentle breaths, you might feel light headed or one of the other symptoms characteristic of hyperventilation. If this occurs, it means that you are working too hard at the exercise. Take smaller and more relaxed breaths.
After you can breath with your diaphragm while lying down, practice two or three times a day while standing. From now on, whenever you begin to feel anxious or panicky, check your breathing. Taking three or four slow, relaxed breaths will make you less anxious.
Source: Anxiety, Phobias, & Panic by Reneau Peurifoy