Here is a very simple technique for shifting your nervous system from fright-flight into relaxation anytime, anywhere. My training in this technique has been through Yoga study, although I do not know its exact origin.
Simply stated, one works consciously with the inhale and exhale in this way:
Breath in to a count of three, hold for three and breathe out to a count of six. You can use other counting numbers by doubling the count on the exhale.
Why does this work?

Within our autonomic nervous system, (which governs involuntary action as in intestines, heart and glands) we hold both the Sympathetic and Parasympathetic divisions.  The Sympathetic Nervous System is connected to alertness and the fight-flight, or stress, response.  The Parasympathetic Nervous System, in turn, is connected to the relaxation response.  By consciously regulating the length of our inhale or exhale we can successfully master the mood!  Read on for more detail.  Some of the jargon is medical-ese.  Just take what helps you and leave the rest.  We can revisit this universally interesting suject later for more simple techniques. 

The following is a clear explanation by Physical Therapist Bill Gallagher from his site everything-yoga.com

The Autonomic Nervous System & Respiration

Breath control is shared between the autonomic and somatic divisions of the nervous system. As such, it provides a way to manipulate the sympathetic and parasympathetic drive consciously. To elicit parasympathetic activity and the relaxation response:

1) Prolong exhalation.
2) Use a diaphragmatic pattern.
3) Slow down the breath.

To stimulate the sympathetic nervous system, increase alertness (and move toward a fight or flight response):

1) Prolong inhalation
2) Use a Thoracic or Clavicular pattern
3) Breathe rapidly

Every psychophysiological process that successful rehabilitation depends on is hampered by a chronic stress response:

1) Muscle hypertrophy: dependent on growth hormone secretion that is diminished by the stress response.
2) Motor control: mind chatter hinders attendance to critical features of the environment.
3) Wound healing: blood flow to the skin is reduced, blood sugar is elevated.
4) Pain management: the stress response increases muscle tension which usually leads to more pain.
5) Energy conservation: the excess muscle activity wastes energy.
6) Cardiac: increased arrhythmia, coagulation, blood pressure and heart rate.

Breath and the Mind

Breath can also be used as a focal point to facilitate an expansive mode of thought. Eastern philosophies see the mind as a powerful tool that, too often, is misused. Rather than letting the mind drag us (or our patient) to lots of scary places, mostly imaginary, we can focus on the breath to anchor ourselves in the present moment. Most of our anxiety has to do with thoughts about the past or the future. By using the sensation of breath as a focal point and letting go of thoughts as they arise, the fact that my thoughts are not reality becomes clear.

Myofascial Function

Breath also has a strong effect on body mechanics and myofascial function. A key component of many head, neck and shoulder pain syndromes is secondary inspiratory muscle overuse. These muscles, including sternocleidomastoideus, scalenes, pectoralis major, pectoralis minor, serratus anterior, serratus posterior superior and upper iliocostalis, are often used inappropriately for “relaxed” breathing. When these muscles are involved in every breath, breathing is far from relaxed. In a similar way, since the diaphragm does not move much in this respiratory pattern, Quadratus Lumborum and Psoas will have a tendency to develop myofascial dysfunction. Since this mode of breathing spawns numerous trigger points throughout the body it makes sense to directly address the myofascial dysfunction via manual therapy. It also makes sense to address the root cause of the disorder by teaching correct breathing mechanics.

Mechanical Function

Lumbar and pelvic stability appear to depend on optimal coordination between diaphragm, the pelvic floor and transversus abdominus. If the diaphragm does not push down on the viscera on inhalation, then transversus abdominus and the pelvic floor muscles are continually in a position of active insufficiency. In other words, these key abdominal and pelvic muscles of stability are kept in too short a position to do their jobs efficiently. If you are teaching transversus strengthening exercise without the foundation of a proper respiratory pattern, your outcomes are less likely to be positive. The diaphragm should not be thought of as solely a muscle of respiration. It is a muscle of stabilization as well. In the same way, don’t pigeon hole the pelvic floor muscles as simply muscles of continence. These muscles also play a key role in stabilizing the pelvis which, after all, is the foundation of the spine. It is quite common for low back and sacroiliac pain to coexist with stress incontinence and again, breathing patterns are the common thread running through these two disorders. Normally, the pelvic floor should move in coordination with the diaphragm. As the diaphragm pulls air into the lungs, it pushes down on the viscera, which in turn, press down on the pelvic floor to stretch it down. On exhalation, the diaphragm and pelvic floor both elevate. When thoracic respiration is the dominant pattern, the pelvic floor does not move. This lack of movement turns the pelvic floor into a “Johnny one note.” Rather than continually moving through its range of motion, it stays in one position all day long. This tends to exacerbate myofascial dysfunction that drives trigger point activation and lumbopelvic mechanical derangement. If we simply address the myofascial and joint dysfunction directly through manual therapy, we are really only doing half the job. To complete the job, we need to address root causes, which often include respiratory habits.

What causes non-diaphragmatic respiration?

1) Surgery & Trauma
When the diaphragm is pulled down to inhale, it presses down on the viscera which, in turn, press down into the pelvis, back into the flanks and forward into the abdominal wall. Normally, this gentle pressure provides a mild massage that can improve digestion, elimination and myofascial function. After surgery or trauma, the intra-abdominal pressure causes pain on a diaphragmatic inspiration. This noxious feedback encourages shallow breathing with minimal diaphragmatic excursion. Just as a limp can continue long after the foot has healed, this shallow pattern can become perpetual.

2) Sinus issues can also affect breathing patterns since diaphragmatic respiration is facilitated by the resistance provided by the sinus passages. When mouth breathing is the only option, a thoracic pattern is likely to follow.

3) Vanity can also drive a pattern that avoids abdominal movement consciously. Tight clothing can prevent optimal breathing.

4) Poor abdominal tone can discourage diaphragmatic respiration by making it less efficient.

5) Chronic mental stress and anxiety, by increasing sympathetic drive, can perpetuate a thoracic respiratory pattern.

Got all that?  If not, that’s ok.  Controlling the length of inhale or exhale will affect the sympathetic (fight-flight)or parasympathetic systems (relaxation response) respectively.  Just play around with it and see what you feel in your own body.

I’d love to hear back from you about this one.