Slow,controlled breathing is good advice Dominique.
Also, try breathing in through your nose and out of your mouth. The restriction of the airways through the nose provides a faster, more powerful delivery of oxygen to your lungs and the larger airway breathing out allows for quicker exhaling. Think of a garden hose with a small diameter sprayer going in and the sprayer off going out. Good luck.
Also, hold shoulders back which opens the lung cavity making it easier to breath (the lungs don't have to "push" the cavity open if it already is). Hope this helps.
