Once you're in a full blown panic attack it can be difficult to stop.
Breathing exorcises are a good start, and practicing meditation can reduce anxiety on a broad level. Plus, once you practice both, you can work with breathing and meditating to try to calm your anxiety much more easily. They don't have to be done in specific circumstances. Just look to quiet your mind, and seek the same state of mind you find in better circumstances.
If you work with energy within your meditation, you can also use this to try to balance out your energy when you are reaching a state of panic or at a level of anxiety where a panic attack seems about to come on.