Breathe. Exhale. Repeat.
The importance of controlled breathing cannot be overemphasized and is an immediate way to help calm the nervous system. Regardless of what's going on in one's surrounding environment, practicing controlled breathing is a great way to relieve stress and feel more centered in one's body. Whatever the feeling in the current moment, whether depression, anxiety, grief, fear, stress, or any other emotion, a few controlled breaths can have an instantaneous effect on one's ability to feel more calm.
The New York Times published a wonderful article explaining the health benefits of controlled breathing along with instructions on basic breathing exercises to try. You can find a link to the article here.