I have asthma, and it's very much under control. The only time it really scared me was when I was selling vacuum cleaners (or rather, trying to).

One thing I've read is that although it seems like you're having problems getting air in, the real problem is you're having trouble getting air OUT. When it comes time to pull air back in, your lungs are still full, so it SEEMS like you're having trouble getting air in. I've experimented with that when I have a bit of a problem while jogging- I'll work hard to give my lungs a bit of extra squeeze at the end of exhaling. It seems to help a bit. What helps me more though is to wear a heart-rate monitor, and use that to pace myself better and not let the asthma activate.

I don't know how this information will help with someone else though, this is purely how my lungs work.