Tips to improve underwater air use.
1) Do not go under until breathing is under control. Many people swim out to a location, or are nervous and breathing fast when they start to descend. You'll burn up tons of air doing this.
2) Do not over exert yourself. This will only increase your breathing rate and you'll waste air. There's rarely a need to kick fast or zoom around.
3) Move slowly and deliberatly. Ties into number 2. When you are slow and deliberate, you need to use very little air, and you are calm.
4) Stay streamlined and horizontal. Again, part of number 2. Less water resistence means less work, which means less air. Stuff hanging and dangling everywhere means less bottom time.
5) Breathly slowly and deeply. This doesn't mean suck as much air in your lungs as you can, that's dangerous, simply take slow deep breaths, as if you are trying to calm yourself down. This does keep you calm, which means less air use, and the breathing itself is efficient for air comsumption.
I don't have many dives yet, 19, but I already hit NDL limits long before my air is up. Most of my dives now end with me having over 1000 psi left. My first dives, I'd be at 2000 psi by the time I got to the depth I wanted, because of fast breathing.
Avoid fast breathing and your air use will automatically decrease.
Aerobic excercise will also help, by allowing you to do more excerise without increasing your breathing rate. That's still my biggest hurdle. If I have to work hard against a current, my breathing rate shots up too much.
Xanthro