Deep Breathing
If you find yourself in this situation, take a moment to breath deeply. Using your diaphragm to breath increases the oxygen in the body and nitrogen which improves concentration and decreases anxiety.
Envision a balloon in your stomach. Each time you breath in, this balloon inflates, then deflates upon the exhale.
For many people, the shoulders rise up on the inhalation. This is a short, shallow, and nervous way of breathing that actually induces tension in the body.
Begin practicing diaphragmatic breathing laying down so gravity is decreased. Feel the belly rise on the inhale and fall on the exhale. Then try this in a chair leaning forward imagining an inter-tube around your stomach that inflates on the inhale and deflates on the exhale. Once this becomes easy, try breathing in standing or in the saddle. In the saddle, breath into the sit bones. To calm nerves focus on a short inhale and long exhale. To increase energy, take a long inhale and a short exhale.