Trying to reduce your air consumption by controlling your breathing is like trying to *push* a rope. It just plain doesn't work.
As has already been mentioned, weighting, trim, and general relaxed comfort in the water are what you have to work with. I spent quite a bit of time in my parents' pool just enjoying being on scuba (in less than a three meter cube of water). That got the relaxed, properly weighted part down. That got my air consumption to not-bad levels, and I was quite at home in the water.
Then in the middle of a spring-diving trip, I bought an extra cam band so I could move about 1/3 of my lead to the shoulder of my tank. When I came up from the next dive, I calculated my surface air consumption numbers (as I had been doing since I was certified) and discovered that I had cut my SAC by a third! That was considerable enough that I figured I should gather more data, but as I came up from each dive and calculated my SAC, I found that it was just as stable at the new baseline as it had been at the higher baseline before I fixed my trim.
In a normal healthy person, your breathing is controlled by carbon dioxide. Burning calories generates carbon dioxide, whether due to physical effort, stress, or any other reason. (I can document a significant difference in air consumption on comparable dives differing solely by the wetsuit I'm wearing. In the chilly springs, the thick, warm suit will always give me better air consumption than the old, thin suit, as the thinner suit means I lose more heat, which my body needs to replace... by burning additional calories.)
To optimize your air consumption:
- Dial in your weighting
- Trim yourself so you're balanced when horizontal
- SLOW DOWN (drag losses increase with the square of velocity, after all)
- Stay warm
- Relax
Forget about your breathing. Your body's been handling that well for however long you've been alive, so don't try to fix it now. Obviously, if you're stressed and hyperventilating, think about your breathing to make it slower and deeper, but in any normal situation, thinking about breathing just means you're doing it wrong. :biggrin: