Breathing correctly before, during, and after physical training can drastically decrease stress and improve flexibility and mobility. Proper breathing allows for the central nervous system (CNS) and peripheral nervous system (PNS) to function at optimal levels. Given the amount of stress related illnesses in our culture, it is believed that we are not breathing very well.
Proper breathing involves breathing deep in your diaphragms, which is called diaphragmatic breathing. Here is the rundown on how to practice this type of breathing:
The benefits of deep diaphragmatic breathing can only be realized with consistent breathing work. Twenty reps a day is a good start and some consider this meditation time.
Fore more learning about breathing, check out this article of Brett Jones with Functional Movement Systems.