There is something called Buteyko Technique that helps asthmatics control their condition. From what I have seen, it can be very effective; for example, allowing an asthmatic to stop an asthma attack.
They have a
simple nose clearing technique which I have seen used very effectively although I haven't used it myself (yet). You just pinch your nose and nod your head gently a few times. There are some slides of it on-line (first 4 slides cover it):
Buteyko Breathing Technique: A Demonstration » SlideShare
More info. on Buteyko at:
Buteyko method - Wikipedia, the free encyclopedia