My therapist taught me square breathing and she suggested starting at the first symptom of panic. If you can remember what symptom you usually get first it makes it easier to try to remedy the attack before it gets so bad. Square breathing is that you imagine a square in your chest and every time you inhale you blow your chest out and fill the square with air and every time you exhale you suck in and release the air from the square. Its hard at first so practice when you are not experiencing an attack. Once you get it down you may realize its pretty effective deep breathing.