While deep breathing (in through the nose, out through the mouth and using your belly to breath) is effective there is a better way. Learn
Deep Muscle Relaxation. It takes four to six weeks to learn well. When it has been learned you can truly calm yourself down in less than 30 seconds. It can be used at any time before or during a match and will promote your best game better than just about anything else that is legal.
Deep muscle relaxation is used by therapists in the treatment of several different problems. It is also an effective technique to use in anyone's life.
You can purchase books and audio tapes that will guide you through the process but it does take some dedication during the time needed to learn to use it. It is like learning to ride a bicycle, once learned you can use it for a life time. The technique is subtle and no one will know you use it as needed.
A reasonably complete description is given at the end of this rather lengthy article kindly republished for me by Dr. Dave. See Below.
http://billiards.colostate.edu/PBReview/Competitive_Anxiety.htm
Here is a quick link to a general description of the procedure. Note that you must do these exercises twice a day for about one month until the muscle relaxation is automatic. I give some practice exercises that are useful for pool players in the article.
http://www.wikihow.com/Perform-Progressive-Muscle-Relaxation
The technique is also known as "Progressive Muscle Relaxation."
The most powerful way to learn these techniques is through the use of bio-feedback and under the guidance of a professional. However the basics, that will change your life are easily learned.
For those who don't know me,
Joe Waldron, Ph.D., psychologist (retired).