ScubaRich suggested riding a bike. He’s got the right idea, but since riding a bike isn’t for everyone (it positively kills my backside), any form of regular exercise that gets the lungs working will help. If you want something low-impact, try yoga. It’s actually a real workout, with emphasis on breath control. You don’t just sit there with your legs crossed chanting "ohm". (If you’re interested, e-mail me about it. I have more advice about yoga and finding the right instructor.)