ignatiusmael said.

I am going to add that the breathing trick I use is when
inhaling, inhale what you need, then keep inhaling, slowly
until you can feel you can't inhale any longer, then slowly exhale.

Breath in, continue sipping to help relieve the feeling to need to
breath (dilutes the CO2) until your lungs can't take in any more,
then exhale slowly. Do this rythmatically. This along with
your bouyancy (horizontal as much as possible), streamlined,
trimmed and weighted correctly, have a good performing regulator
and time will do the trick.
Also, sleep well the night before, eat a healthy breakfast and lunch,
drink plenty of water and you should be good to go.

Regards,
L