Breathing exercises are an ideal way to relieve stress in that they are fast, simple, free and can be performed by just about anyone, anytime. These factors make stress relief breathing exercises one of the most popular and convenient tension tamers.
How to do:
1. Sit or stand in a relaxed position.
2. Slowly inhale through your nose, counting to five in your head.
3. Let the air out from your mouth, counting to eight in your head as it leaves your lungs. Repeat several times. That's it!
Tips:
1. As you breathe, let your abdomen expand outward, rather than raising your shoulders. This is a more relaxed and natural way to breathe, and helps your lungs fill themselves more fully with fresh air, releasing more "old" air.
2. You can do this just a few times to release tension, or for several minutes as a form of meditation.
3. If you like, you can make your throat a little tighter as you exhale so the air comes out like a whisper. This can add additional tension relief.

Source: http://stress.about.com