I also have wicked asthma. Some seasons are worse than othes. I hit my inhaler about 5 minutes before I ride, especially if it's cool out. I make sure I'm using my other meds religiously.

However, sometimes it just happens. That tight, scratchy feeling. Then the cough. When I do that I just follow the advice of Mae West "put you lips together and blow". Purse your lips (like you're blowing out a candle), and blow steadily out with every exhalation until the feeling subsides. If you can slow your pace a bit at that point, it will help too. This is a technique we teach people with COPD. The big problem with asthma isn't that you can't get air in, but that you can't get CO2 out. Pursing your lips creates a back pressure in the lungs, helping to force the CO2 out of the base of your lungs. Inhale slowly and repeat. You really have to focus on the breathing at that point, and avoid the panting/coughing panic that accompanies the attack.

If I know I'm getting to a point where I'm really going to be sucking wind (ie, working hard) I just start blowing slowly. It seems to prevent those 'oh, God, I'm peeing my shorts' coughing fits that go with my asthma attacks.

Good Luck