I was watching a training DVD on open water swimming in triathalons (just as it came free with a magazine I bought, not something I do!) and the guy talked about breathing in slowly through the nose, and out slowly through the mouth... BEFORE activity. Ideally getting this down to four inhales/exhales per minute.
Idea being it's the best way to calm you and flood/charge your system with oxygen... though it's certainly not going to be the case when your body is working out intensively though - your nose simply cannot supply enough air to the lungs!