I have found the following coping skills particularly helpful in managing PTSD panic attacks in public (in order from most to least). Please note that not all of these coping skills will be helpful for you.
I. Freeze a bottle of water, and take it with you when you leave the house. When you begin to panic, place it over your sinuses for at least 30 seconds (an ice pack works better, but a water bottle is more discrete). Focus on how it feels.
II. Deep breathing exercises (it can be helpful to carry a paper bag along with you in a backpack, purse etc).
III. Carry intensely flavored gum with you, and chew it when you begin to panic. The intense flavor can help ground you. (I prefer peppermint). Focus on the taste.
IV. Carry scented lotion with you. When you begin to panic, rub it on your hands. Smells can help to ground you. Focus on the scent.