Try the "s" breathing technique. Breathe out slowly, hissing on the exhale to where you feel your lungs almost empty completely. Don't actually leave yourself with zero air. Then take a deep breath in. This actually helps to increase your air capacity, it slows your breathing, tones your diaphram, and the added oxygen increases "feel good" chemicals that reduce stress. Be sure to breathe in slowly and deeply. You'll actually feel your lungs receiving more oxygen. To me it feels great.
After you've done this for a minute or two, you're mind is probably blank because you're so aware of your body.
Now you can breathe in through your nose and out through your mouth with full normal breaths. When using breathing techniques, I like counting the seconds of my exhale and inhale. I take breaths that feel comfortable, and don't try to conform to a certain amount of time. But my exhale time always equals my inhale. The key is to regulate the airflow and create a rhythm. My breath naturally regulates to a certain amount of time (5-8 sec). Never exhale to a point of discomfort. Exhale to the point of feeling the need to inhale. Eventually you will naturally be able to exhale more.
Eventually I stop counting and just enjoy the moment. My breath continues to be rhythmic and even. My body is relaxed and my mind is quiet.
I always use breathing techniques to unwind after a long day along with yoga followed by meditation. It's my ultimate stress-be-gone method. =)