Any time you're experiencing stress, it's actually a triggering of your fight-or-flight mechanism. That may seem like a bit much, because you're probably only thinking consciously of whatever triggered your stress to begin with. But, at the deepest level of your subconscious mind, the part known as your primitive mind, someone you trust implicitly – yourself – is telling you that you are about to die, and you'd better do something right now to save your life.
Once stress (or fight-or-flight or anxiety) is triggered, several things change within you physiologically in order to accommodate what your primitive mind believes is about to happen. Your appetite might turn off or become stronger. Your breathing and respiration will likely grow shallower and more rapid. Your body will release stress hormones, like adrenaline and cortisol, which leaves you with an awful, dreadful feeling. And the blood in your brain will move from one part to another, causing you to have difficulty focusing. All of these things will happen automatically – without your permission – below the level of your awareness.
Since one of the changes that occurs is that your breathing becomes shallower and more rapid, the easiest, most-effective technique you can use to counteract stress is simply to take in long, deep, slow breaths. By "deep," I mean imagine drawing the breath from your mouth or nose down to the very bottom of your abdomen, the area below your navel.
Now, if you have even a cursory knowledge of human anatomy, of course you know that your lungs don't go down that far; they stop above the bottom of your ribs. However, if you imagine drawing the breath down to the bottom of your abdomen, what happens is that you engage your diaphragm, which causes your lungs to fill properly.
Normally, when you're not paying attention to how you're breathing, if you're just sitting at your computer or watching TV, for example, as you breathe, your chest and shoulders rise and fall. That's enough to keep you alive, obviously, but it's a shallow breath. Since one of the changes that occurs when you're stressed (or anxious or in fight-or-flight) is that your breathing becomes more shallow, you have a strong, subconscious association to shallow breathing and stress. That means, whenever you're breathing shallowly, you're far more likely to be triggered into stress at the slightest thing.
So, give yourself this gift: Print the "Breathe" sign from http://www.relieveyourmind.com/Free-Stuff.html. Fold it in threes on the lines and place it somewhere you will see it frequently. When you see it, practice breathing to the area below your navel. You may only do it five or six times before you get distracted and return to breathing shallowly, but at least you're practicing, which is great. Every two or three days, switch to a different side of the sign so you don't get to used to seeing it there.
If you're having difficulty breathing properly, it can be helpful to place your hand on the area. It's also quite helpful to lay back to do it.
Be willing to practice this. Not only will you find that, more and more often, you're breathing properly, but you'll also realize at some point that it's becoming easier and easier for you to get through stressful situations more easily and calmly.