From the March 2015 issue of Real Simple magazine, "Breathe" by Margaret Townsend:
When you’re overloaded with stress, you tend to breathe in a shallow way, not fully emptying your lungs. This just creates more tension in your body. Lengthening your exhale can stimulate your body’s ability to relax and release stress.
Take a breath, then exhale with the feeling and sound of a sigh of relief: “Aahhhh.” Notice how your chest softens downward under your hand as air flows out. Do this a couple of more times and think about your lungs releasing toxins and stale air, creating more space for fresh air.
Then inhale, and on the exhale whisper, “La, la, la, la…” for the length of the exhale. (This helps carry air out of your body.) Feel the air emptying out.
Let each inhale come naturally, whenever it arrives, allowing your abdomen to soften each time. Repeat until you feel your whole body softening.
How are you feeling? You may notice your breath emptying more completely, without any force. Enjoy this, following each breath all the way to the end and as it comes in effortlessly. Then let your hands rest in your lap and your body and mind rest in the nourishing space you just created.