To go along with gwmccull's advice on nose breathing....
When I started to run over a year ago, I would get these incredible, 'stop me in my tracks' chest pains. They got so bad that I was on the verge of a cardiologist visit. That was, until someone suggested that I regulated my breathing by going through my nose, out my mouth. I said how ridiculous that was....how could pains that paralyzed me mid run be corrected by simply breathing through my nose? So, I changed my breathing and voila! No more chest pains. I also learned that when my breathing would get out of control, I'd walk until it was, then start running again.
Another advantage of nose breathing is avoiding dry mouth.
Two running books I have suggest to breathe any way that is normal and comfortable to you.....in nose out mouth, in mouth out mouth or any combination, as long as it's done in a controlled manner.
Find what works for you and go with it.