@maxineinyourwifi @gabboman I read recently about breathing for five counts in and another five out to activate the parasympathetic nervous system (the one that calms you down). The important part is to breathe out for slightly longer than you breathe in, to expel carbon dioxide. Breathing deeply helps too. The slow pace is to avoid hyperventilation I think but anything slower than normal should be enough.
And if you're in a panic attack type situation I would suggest self-distraction, too.