Your breath can be a useful tool to achieve a relaxed clear state of mind. Keep your emotions in-check and remain calm under pressure. When we are stressed we often use only the top third of our lungs but it's the bottom third that supply's most of our capacity. Use this simple technique.
Inhale through your nose until you feel your belly expand
Hold that breath while you count to six
Exhale slowly through your mouth relaxing your face, jaw, shoulders, and stomach
Repeat until you are calm