The simple breathing technique: breathe in for count of four, hold for four, out for four. Or in through the nose for a count of four, hold for seven, out slowly through the mouth for a count of eight (as though you are trying to make a candle flicker).
Another good one is to tense every muscle then release it. Work through the body: tense feet, relax them, calves, relax them, thighs, relax them, bum, relax, stomach, relax, shoulders, relax, arms relax, fingers, relax, face, relax.
Do this once (breathing normally as you do it) then a breathing exercise 3 times, then the muscles once, then breathing exercise 3 times.
If it really isn't a life or death situation, try giving yourself little mantras like: 'I can handle this,' or, 'This too shall pass,' or, 'This won't be happening in two weeks time, or This won't matter in a thousand years, a hundred years, fifty years, ten years, five years, two years, a year, six months.' Work backwards through a big period of time until you realise how small a period of time the stress is for and how little the world cares, in the grand scheme of things.
It might also help to say to yourself: This is happening anyway. I can feel stressed about it or I can let go of the stress. I can't control what's happening but I choose to control how my brain and body respond. I choose to stretch, to walk, to listen to music, to play Candy Crush, to make something creative, to plan for better times when this is over etc etc.