Every year it seems more people I know have asthma. I'm not sure if it's age, diet, environment, or something totally different.
Regardless there is a quick breathing technique that can be very helpful:
Inhale through the nostrils and take long slow exhales through the mouth with a straw. The inhales remain through the nose, but each exhale imagine a feather on the end of the straw and breath out so smoothly and slowly that this imaginary feather barely moves. Use the abdominals to gently lengthen the out breath. Then release the belly and let the inbreath come in naturally. Do this 10-12 times.
Many people diagnosed with asthma focus on the inhale which is the first mistake. An inhale has nowhere to go if a full exhale hasn't preceded it! So exhale deep and long first. This will also slow the heart rate and lower anxiety which easily aggravates an asthmatic condition.
Finally, when inhaling don't force it; just relax the belly and the breath will deepen as the exhales deepen.
By the way, this technique is wonderful for anyone looking to calm down and release tension.
Best of luck!