I think "take a deep breath" is often bad advice (subjectively, for myself). I find that holding my breath on empty lungs helps calm me. I'll hold until I feel I absolutely must breath, then take in a gentle breath, hold until I've quenched my thirst for oxygen (but preferably briefly), push out again and hold on empty lungs, repeat as many times as necessary. Battling my primal urge to breath, however momentarily, seem to put other issues into a frame of reference. Most things bear much less significance than the urge to breath.