An Indiana University study found that strengthening inspiratory muscles by performing daily breathing exercises for six weeks significantly reduced the amount of oxygen these same breathing muscles required during exercise, possibly making more oxygen available for other muscles.-ScienceDaily
Inspiratory Muscle Training (IMT) in this particular study involved forceful inhaling through a hand held device 30 times, twice a day, for 6 weeks. While researchers did not actually demonstrate an improvement in athletic performance, "when the study participants mimicked the breathing required for low, moderate and maximum intensity activities, the inspiratory muscles required around 1 percent less oxygen during the low intensity exercise and required 3 to 4 percent less during the high intensity exercise."
This study Training Breathing Muscles Improves Swimming Muscles' Performance reported "improved respiratory endurance and surface and underwater swimming times by 38 percent and 26 percent, respectively" in subjects who underwent 30 minutes of IMT five times weekly for 4 weeks.
That makes me think I could benefit from this training because try as I might, I seem to use up air faster than most when scuba diving.
So other than maybe sucking hard through a straw a copule times a day, what else can you do to strengthen your breathing muscles? How about playing the harmonica?