An instinctive movement, arching is a great way to loosen up your back. When added to forward bending and side bending, it allows you to stretch your back in any direction while standing - handy if you are somewhere that prevents you from lying down (for example, at work).
If you watch a dog or cat when it wakes up, the first thing it does is a full arching movement - replicating this is not a bad way to deal with the stresses of sitting too long in a car or at a computer.
Performing this stretch is easy. Stand with the palms of your hands against your butt, then move your hips forward and lean back as far as you can comfortably go.
Hold for a few seconds, genlty exhaling, then straighten back up. Relax for a moment then repeat. Doing this several times a day throughout the day will help keep your spine flexible.
If you find this a little tricky, or you have a very limited range of motion to begin with, just stick with it, and in time it will get easier.
As with all stretches, you should move to the point of tightness, but never to the point of pain.