I read a post today that compared whether taking deep breaths or  shallow breaths were best during an anxious or stressful moment.  It is my opinion that the depth of the breath is irrelevant. It is focusing on your breath that helps to calm you. If you could turn your attention to focusing on the inhalation and exhalation of your breath, you will be calmed. This is a result of the fact that you are turning your attention inward, quieting your mind and releasing your focus on that which has caused you anxiety and/or stress.

Focusing on your breath is one of many tools you can use to help you when you are experiencing an anxious or stressful moment.  If this is one of the tools you like to use then practice it on a regular basis. When you practice it on a regular basis,  you will have trained yourself to instantly do this and it will become easier and easier to relax and calm yourself.  You will also be bringing an increase in the level of consciousness into your life.  Among many things, this means you will become more self-aware and you will be able to stay more relaxed and less anxious for longer periods of time.