The fight-or-flight response was quite useful in the distant past, when most of the threats humans faced were threats to their physical safety - dangerous animals, or warring tribes, for example. Fighting or fleeing were the best ways to ensure survival. In contrast, the vast majority of the threats we face now are not threats to our physical safety. Instead, we experience anxiety about things like money, obligations, being judged or evaluated, etc. Neither fighting nor fleeing is very productive for these modern-day threats!
Indeed, the physiological responses designed to prepare our bodies to fight or flee can actual interfere with adaptive responses to today's threats. For one thing, it's very hard to think rationally when the sympathetic branch is activated, making it difficult to navigate the complex social and logistical challenges we perceive as threats. Additionally, our common threats tend to last longer than a physical threat - for days, weeks, months, or even years, rather than moments. Chronic exposure to stress hormones, and activation of the sympathetic branch can result in serious health problems: high blood pressure and other cardiovascular problems, reduced immunity to illness, inflammatory problems (e.g., irritable bowel syndrome, asthma, psoriasis, lupus), pain, sleep disturbance, reduced memory and concentration.
Fortunately, we can increase the efficacy of our immediate response to threats, and reduce the health risks of long-term stress by activating the body's opposite process: what Benson and Klipper dubbed the "Relaxation Response." This response is based on the second branch of the autonomic nervous system, the parasympathetic branch, which triggers effects opposite to the sympathetic branch. It reduces heart rate, makes breathing slower and deeper, increases digestion, reducing muscle tension, restoring body temperature and immune functioning, etc. Since both branches cannot be active at the same time, activating the parasympathetic branch deactivates the sympathetic branch, short-circuiting the fight-or-flight response.
That sounds good, of course, but how do we activate something that is involuntary, and therefore not in our conscious control? While we can't intentionally alter our heart rate, for example, we can begin to influence the autonomous nervous system through body functions we have some control over. Take breathing, for example: while it (thankfully) happens automatically, we can alter the rate and depth of breathing. Intentionally taking slow, deep breaths is one way to activate the relaxation response. We can't directly affect our body temperature, but we can use hot or cold compresses to affect temperature indirectly. We also have control over most muscles. Using strategies like progressive muscle relaxation, we can reduce muscle tension and related pain. Any and all of these strategies can counteract fight-or-flight, trigger the full relaxation response, and therefore lessen distress and improve the effectiveness of our reactions.
It's obviously much easier to activate the relaxation response from a neutral state than in the midst of fight-or-flight. Since it gets easier with practice, it's a good idea to practice relaxation in the absence of a threat, so that it's available as a coping strategy when threats occur. (Of course, should you find yourself facing a bear, or other physical threat, let your fight-or-flight response protect you!)