What can I do to calm myself down during a panic attack?

Vagal toning. Diaphragmatic breathing, as taught in the martial arts and certain yoga pranayama exercises, can work more effectively and far faster than any drug. For long term improvement try various parasympathetic toning strategies...Tm, mindfulness, yoga, holotropic breathwork, healing breath, acupuncture, jogging, tai chi.
While it's happening. When ur brain senses danger (even when there's none) its fear & rage centers go into overdrive preparing ur body 2run or 2fight. The centers get u 2breathe fast because ur body needs lots of oxygen. (o2) 4 running/fighting. But if u get too much o2 in relation 2carbon dioxide (co2) u experience sensations of panic. To chase away panic:, take slow, shallow breaths. Cup ur hands & breathe thru them.
Meds breathing. Medicine, deep breathing, relaxation techniques, remember it is a panic attack which will subside & u r ok.
Panic. Please refer to my previous answers on panic for details.