I, too, have had air issues almost all of my diving history (14 years now), and the following do work for me:
1. Exercise including relaxation with (of all things) yoga. Cardio swimming as FtMyersTom describes, and then routine yoga as well. I find that when I roll off the boat I am ready to be calm and breath slowly as practiced in yoga, in a meditative state of mind, reducing air consumption. However, if I find the surface choppy, a long swim back on a shore dive, or some other cardio demand, it is available to me. I am locating snorkle spots that allow me to test equipment and get some serious swimming in - I find regular pools confining.
For deeper dives than 50 ft:
2. Consider diving with a 100 cu. ft. tank. If steel, be certain to adjust your weight. Call around to the shops to find one that has them, you may need to reserve well in advance.
3. Do get a Nitrox certification as well. There are dive you will appreciate the additional O2, but it is not for every dive.