I wrote a while back about the power of tapping into the simple practice pausing and breathing. This is a topic worth coming back to a few times to fill in details, because it can be so essential to living in today's world.
Our autonomic nervous system is divided into two branches. One of the branches, the sympathetic nervous system, is basically the accelerator for the fight or flight response. When we encounter stressful situations this portion of our nervous system is engaged. Parts of our brain out of our conscious control begin the chemical cascades that pour adrenaline and cortisol into our bloodstream preparing us for action, as if we were going to be doing actual fighting or running for our lives. This is exactly what is needed to either fight off an attacker or run until we're out of immediate danger. It is far less helpful when our sympathetic nervous system is aroused by a looming deadline or by the frustration of arguing with your child over chores that aren't done. In fact our sympathetic nervous system being "on" over time, as in unmanaged stress, is a known slow but predictable killer. Fortunately we have another part to our autonomic nervous. If you have an accelerator to propel the car you've got to have brakes to slow or stop it. Just like the sympathetic nervous system, the parasympathetic nervous system has connections all over the body (pupils, blood vessels, heart, lungs, etc), but its function is to slow our body systems down. So when you're really relaxed, your parasympathetic nervous system is the one running the show.
Pausing and breathing deeply and slowly can be like our secret dimmer switch to the autonomic nervous system. Whenever you find yourself edgy, nervous, cranky or anxious that can be your cue to do some breathing. You might want to road test it right now. Get comfortable where you are sitting, and take some nice slow, deep breaths through your nose. As you settle in to the breathing, each breath will tend to come a bit more slowly and last longer. You can put one hand on your chest and one on your belly to monitor which area is moving most. You want your belly to be moving at least as much as your chest if not more. Chest breathing is like pressing on the accelerator where belly breathing the way to ease on the brakes.
This is worth experimenting with for two reasons. First it is good to try this out and actually experience how your nervous system, and your body is able to slow itself down. There is no replacement for actually feeling yourself what it is like to breathe your way from a state of agitation to calmness. Second, by getting experience with deep breathing, you also train your body to get to a calm state more reliably, and more quickly. The main point is that you don't want to wait until you are in the middle of a very stressful situation to think about using that dimmer switch. You want to have some experience with it ahead of time.
Many find that breathing combined with mindfulness practices can be an even more effective route to becoming calmer, but if that's not your cup of tea, the breathing works fine on its own for purely physiological reasons. You don't have to buy into any particular world view to enjoy the benefits of pausing and breathing.