You don't win boxing matches in the ring. You win them on the road and in the gym, building the strength, skills and endurance you need to make it through the bout victorious. However, all the training in the world won't do you any good if you get winded during a fight. Once your breath starts to come in uncontrolled gasps, you'll lose punching power and your ability to fight smart. Your opponent will be quick to capitalize on the opportunity.
Get plenty of cardio training before the fight. Your trainer can help prescribe a good program to suit your needs, but running, cycling and jumping rope are going to be a major part of your life in the weeks leading up to a bout. Without this cardio training, you'll be unable to keep your breath under control for the rounds of exertion ahead.
Do sprint workouts several times each week. Long-distance cardio is best for building cardiovascular endurance, but sprinting teaches you how to control -- and regain control of -- your breathing while under exertion.
Breathe naturally in through your nose and out through your mouth. If you pay attention to this cycle and rhythm, it can help you focus on controlling your breathing as your body demands more oxygen.
Exhale sharply with each punch you deliver. This keeps you in control of your breathing's rhythm, in a process similar to the "kiai" of eastern martial arts. If you're delivering a punching combination, exhale in a series of pushes with no inhalation in between.
Exhale sharply when you are punched, especially when receiving a body blow. This will put you in control of your diaphragm, which is likely to seize under some punches.