Well I have found a really great source for back stretches diagrams.  The site is actually a fitness site, but it clearly shows how to do all sorts of stretches, and has an animation, diagrams and a short explanation.

The type of stretches include:

Upper Back Stretch – pushing against a wall

Lower Back Stretch – lying on the floor bring your knees to your chest

Hyperextension of Back – lie on your stomach and look up to the ceiling

Spine Rotation – lie on your back and move our knees from side to side

Cat stretch – get down on your hands and knees and arch your back

These are the perfect lower back stretches to help loosen muscles and get rid of your pain. Here it is:

http://www.abc-of-fitness.com/stretching-exercise/back-stretch.asp

Note: You can see an example of the type of diagrams at the top of this post.