I was listening to the podcast "Mind Pump" last night and the guys interviewed Steven Kotler. Kotler discussed a wide range of issues and he touched on deep breathing techniques. The technique of exhaling at a 2:1 ratio of inhaling (for instance: inhaling for a count of 5, then exhaling for a count of 10) actually changes the feedback loop between the body and mind. Regardless of how stressed you may be, focused breathing with long exhales sends a clear signal to the brain that you are relaxed. This in turn changes the state of your mind from the fight or flight mindset to a more focused calm.
As the band Cheap Trick reminded us...everything works if you let it.