Any ideas for stopping panic/anxiety attacks after it has started?

Lay on the floor and put your hands on your stomach and take deep breaths in so you feel your stomach going out. Then slowly let it go. The way you feel is due to too much oxygen, not too little. And it's not going to harm you.

Breathe into a brown paper bag. IN and out slowly. Well, it doesn't have to be brown.

move your mind to something different - put it aside

Breathing exercises and upbeat music

go to your happy place.

Pray

There is no stopping it, your very thoughts is what causes it so trusting your thoughts to stop it would be a waste of time. Just accept that it's happening really.