As Sabby said, breathing is key. I, too, tend to not breath when I'm in that state. Now that I'm better at recognizing the pre-cursor I'm better able to manage them.
When you breathe, use your belly to "pull" the breath down into your lungs by extending your belly on the inhale.

Here are a few other breathing exercises:
Alternate Nostril Breathing:
Gently close your right nostril and breathe in slowly thru your left, release the right and close the left and exhale slowly. Then inhale slowly from the right, release, close the right and exhale. Keep repeating for as long as necessary.

Relaxation Breath:
Breathe in slowly for a count of 4, hold for a count of 7 and exhale for a count of 8. If you are having trouble with this, take it slowly. Some alternatives are to skip the "holding" piece and or exhale for a shorter count, as long as it is longer than the inhale.