Strength imbalance, common for cyclists, in in several areas can cause pain and even injury. Doing regular abdominal excercises like sit-ups or crunches can prevent this problem and even give relief when occuring. Abdominal work keeps the back muscles stretched, and the spine in proper alignment. Weak "abs" allow the back muscles to over-tighten and over-develop, pulling the spine into an unnatural position.
THIS IS TRUE FOR VIRTUALLY EVERY CYCLIST I KNOW WHO DOES NOT DO "AB WORK".

Unequal leg length, more common than you might think, can cause back pain. The back becomes slightly curved to one side shortening and tightening the muscles, on the "short"
side.
The cure is to put a spacer, 1/2 as thick as the leg length difference, in the shoe or between the cleat and the shoe. If your leg length difference is 2cm then shim/space 1cm.

As mentioned in the earlier reply, stretching is very important, however be sure to be "warmed up", do the stretches slowly while breathing deeply, and never, never "bounce" the stretch.
If you can't find stretches in the cycling books in your library, try looking for stretches in martial arts, (karate judo etc.), books or maybe even better in modern dance or ballet texts, no kidding!