Although it's not a natural movement, side bending is a good way to increase the all around range of motion in your back. When added to back arching and the standing forward bend, it will give you increased flexibility in every direction.
Performing side bending is easy. You stand with your hands by your side, fingertips resting against the sides of your thighs.
Slowly slide one hand down towards the knee, keeping your trunk upright and avoiding any twisting as you bend your body sideways. If you imagine you are stuck between two panes of glass you'll understand the motion better.
Reach as far down as you can, pause, then slowly straighten up.
While you will see some people going from side to side, it's probably better to do 5 to 10 bends to one side, then take a short rest before doing the other. While you are resting, you might like to stretch upwards to open your spine up again before compressing it to the other side.
As with all stretches, you should move to the point of tightness, but never to the point of pain.