During intense and enduring workouts it's not always easy remembering to breathe, so I plan breathing intervals (for like 10 minutes) between each segment of my workout.
Breathe in deep through the nose, hold the breath, try to inhale again through the nose (if you can't it's ok), and then exhale deeply through the mouth.
This helps you to remember to breathe, (no kidding) this helps you control breathing, and it improves your breath quality.
(You may find that after adding a few intervals you can breathe better, I did).
I have no idea if this works for everyone, I am breathing easier now during my workouts so...