Having a breathing technique that you use can come in useful to calm yourself down if your feeling stressed or anxious, it only takes a few minutes and can be done anywhere.
This is one of my favs:
Let your breath flow down as deep into your tummy as is comfortable. Breath in through your nose and out through your mouth. Count steadily to 5 on your in breath, then without pausing or holding your breath, let the breath flow out gently for the count of 5. Do this over again for a few minutes