So.... the air you take in is important and good breathing is conductive to good health. Slow, deep breathing exercises the lungs by keeping them pliable. It also focuses the mind and helps reduce stress. Natural breathing helps you in your every day life and during training.
You can practice chi kung exercises, aerobics, or just getting out and about in the fresh air. Sanchin kata is also a useful breathing exercise, as long as you don't force your breath.
Breathing can be improved by working your posture to eliminate excess tension from the upper body, and to give your lungs room to expand and contract. Another tip is to develop a mental awareness of your breathing. Ask yourself "Am I relaxed?" or "Am I breathing from the lower body (centre, tantien or diaphragm); or simply "how's my breathing?"
By directing your mind to your breathing you can immediately begin to change it for the better. There are plenty of other exercises too.
Don't forget to work on your breathing in meditation. A useful trick for relaxing your mind is to count your breathes. You can also do a breathing visualisation, so that now you are "breathing with the mind".
Get into your usual meditation posture. If you don't have one, sit comfortably or lie down (not while tired, you'll fall asleep!) Relax yourself and calm you mind. Focus gently on your breathing, not forcefully or trying, just be aware of what's happening. Don't judge yourself.
Now imagine the air circulating around you - in through your nostrils, down into your lungs and circulating all around before being exhaled. Get used to the sensation, again, without forcing it. Take your time.
The next stage is to imagine your entire body is breathing, absorbing the air through your skin. Imagine floating on air - not only is it around you, but it flows through you, cleansing and feeding your internal systems. Imagine your blood cells absorbing the air and transporting it all around. Really get into the imagination and fill yourself with life giving air.
Once finished, complete your meditation routine, sit quietly for a few moments and carry on with your day feeling refreshed.
I've had asthma since the age of 15, when I first started training. That hasn't put me off doing karate. I think I would be much worse if I hadn't started, so please if you're a beginner, don't use asthma as an excuse to avoid starting classes.
Another interesting tip - another word for "air", is chi...