The main thing is your breathing - breathing deeply. It isn't easy to do when you start to feel a panic attack coming on though. Its the fear that I am going to stop breathing that scares me the most which makes my breathing more shallow. I know breathing is involuntary and it just happens, but feeling that I can't breathe for me is really scary. Saying it is all in your head is not helpful and only people who don't experience it can say that kind of thing.
It is a physical feeling and it is difficult to control.
I try to calm myself by saying "you can breathe, take deep breaths, you will feel better soon" etc. I don't seem to get really bad panic attacks except for about 3 times in the last 6 or so months, one being particualrly bad.
Does anyone get this feeling that they are too tired to breathe and you're afraid you'll just stop? Thats how it gets me and it can feel horrible.