Calm your spirit within, stay focused, don't make unnecessary movements. You don't need to breather through your mouth, that is only for those who have wasted too much energy from their unnecessary movements.
It's not how you breathe, it's the energy you use.
You can also do it the easy way and just increase stamina by running miles and or hitting the boxing gym every single day, hit the punching bag, sparr a lot.
The top one is the mental way, the bottom one is the physical way. If you use both, you will be a force to be reckoned with.
I just made all of this up, I have been watching too much Dragonball Z. I am learning the ways of a Sayin so that I can use their mental toughness in my everyday life.
^Damn even I impress myself sometimes
Honestly I just wrote that off of common sense.