A powerful breathing technique that I sometimes use to center myself is based in Buddism, I believe. I don't know where I learned this, but it is a fast, easy, and powerful way to center, relax, and energize yourself.
Start in a comfortable position either seated or lying down. Breathe in deeply for a count of three, filling your lungs completely, but not until they are uncomfortable. Hold the breath for a count of three. Then exhale completely for a count of three, allow your body to relax and release tension. Then relax for a count of three before taking another breath. Do this for as long as you like, allowing each complete breath to remove the stress from your body.
It is also an excellent meditation technique because your focus is on your breath, which gives the lower self something to do while the higher self does the meditation work. It's a win-win! So next time you are feeling tired or stressed, try this out. You will find yourself lighter, energized, and less stressed. Enjoy!