Don't Think About It!
I had lots of problems when I first started going to my vocal coach, Gerald Arthur. He had to tackle my guarding, my inflexibility, my mix imbalance, my range limitations. When I asked him what he thought were my breath problems he flatly told me something to the effect, "you're breathing is fine; if it weren't I would tell you, until then don't worry about it!" And we never had another discussion about breath.
What happened, of course, was that he tackled my breath issues from another direction. He wanted me to concentrate on other things, and the breath problems resolved themselves without direct intervention.
Thinking about breathing while performing actually causes tension and worsens your breathing problems. Instead, when you sing or speak, learn to habitually stand or sit tall, head balanced on tailbone with your ribcage open.
If that doesn't work, get with a vocal coach who can assess and help you fix the source of your unique problems in one or more of the following breathing techniques:
2. supporting your breath
3. being able to control breath.
Breathing... it's important, and it's important to get so right you never think about it.