Nadi Shuddhi - Recommended by Swami Vivekananda

Take a deep breath and fill the lungs. Slowly throw the breath out. Take it through one nostril and fill the lungs, and throw it out slowly through the other nostril. Some of us do not breathe deeply enough. Others cannot fill the lungs enough. These breathings will correct that very much. Half an hour in the morning and half an hour in the evening will make you another person. This sort of breathing is never dangerous.

Source: The Complete Works of Swami Vivekananda