The trick to any subsurface breathhold swim is your stroke efficiency. You can get away with bad form in surface swimming (and in scuba) where air is plentiful, but you can't with breath-hold. Generally speaking, you want a STRONG stroke (with perfect trim) followed by riding the momentum until right before you come to a stop. if this is in a pool, you want to swim as deep as possible, hugging the bottom of the pool the entire way.
The stroke itself might be quite a bit different with tanks (particularly side mount), but the general idea of using a physically efficient stroke is the same. Strong thrust, ride momentum, strong thrust in order to minimize your cycles and maximize your oxygen. Your stress level plays a significant factor too. The more you swim, the more efficient and less stressed you are, so get in the pool a few times a week and hit some laps.
If understanding physiology helps put you at ease, know that your body does not trigger breathing based on lack of oxygen. It triggers breathing based on CO2 concentration in your blood. You (very generally) still have nearly 2 minutes of O2 left when you start feeling the burn and the panic.