"Tactical breathing."
Four count in through the nose. Hold for four, then out through the mouth for four.
I've used this in some exceptionally high stress situations at work. I've used this to keep the right level of excitement while in the door while competing at the USPA nationals in Canopy Piloting as well as on the swoop tour.
I've used this to keep the right level while getting ready to get on the platform for powerlifting.
I used this technique standing at the alter for my wedding.
It works well!