OK, I am not a streamlined torpedo by any means: 6' tall, 250 lbs. Last weekend, the two dives I did was one dive of about 45 minutes, primarily to and at 60' , started with 2900 psi, out of the water with 845 psi in an AL80. The other was a multilevel, about 15 minutes at 85-90 feet, 15 minutes total about 55' and 20 minutes at 25-30' and the last 5 minutes at 20'. I started with 2850 psi, out of the water with 425 psi in an AL80. I also carry a AL40 pony in preparation for the ocean next year. (increased drag)
I am a very new diver, only underwater a total of 30 times including the 5 training pool dives. One of the things I do is breathing exercises. They are part of relaxation techniques I have used for several years. All I do is practice slow, regular breathing. Slow in and slow out. Don't hold your breath, and don't take large, uncomfortable breaths while trying to relax.
Most of all, relax and have fun. If you slow down your movements, you will conserve energy as well as air. You will be more calm, too.