The NYT illustrates some approaches to controlled breathing:
Controlled breathing…has been shown to reduce stress, increase alertness and boost your immune system. For centuries yogis have used breath control, or pranayama, to promote concentration and improve vitality. Buddha advocated breath-meditation as a way to reach enlightenment.
Science is just beginning to provide evidence that the benefits of this ancient practice are real. Studies have found, for example, that breathing practices can help reduce symptoms associated with anxiety, insomnia, post-traumatic stress disorder, depression and attention deficit disorder.