Wow! Someone else has the same problem! When I get nervous, I sometimes forget to exhale. I breathe in as much air as I can out of fear I'll run out, I think. I have managed at least once to stop it by reminding myself how to breathe.

My friend, who has been "trained" and who's been giving me advice, told me to breath with my lower abdomen. He said my shoulders shouldn't move up, or my chest visibly expand when I breathed, but my belly should pooch out. I thought this sounded like a load of crap, until I tried it, and figured out what it really did.

What it does is allow you to not overfill your lungs, thereby taking the pressure off your upper chest, throat and vocal cords. It sounds weird, but try it.