i struggle with breathing ALOT...i have in fact hyperventalated a few times on the trails (mostly when i have just done something extremely technical and it finally hits me what i have done or when i am mentally drained)
i have been told to try a couple of things...one which seems to just get me refocused on breathing is to slowly count to 10. another which i have not done yet but seems to be a yoga-similar technique...lay down and put a book on your belly...breath in raising the book counting to 5...eventually working your way up to 15....like i said i have not tried this one yet but in theory is sounds good.

the most important thing i learned is from a story about Ulrich...to force your belly out while inhaling, this allows more room inside for your lungs to expand....not the most flattering but if it works who cares!

good luck and do allow yourself warmup time...it usually takes me up to 1/2 hour to feel the zone!