Sit down on a bench or chair, put your head between
your legs, and
force yourself to breathe in deeply, and
breathe out deeply. It's when the breathing gets to short and quick that the panic gets worse. At first you might feel like you can't do it, but with practice, you can.
If you wear correctional glasses, take them off. For some reason sight plays into some peoples anxiety problems.
Ok, now for when you are in an actual panic attack.