Here is a visualization to help release a tight lower back from the book, Human Movement Potential by Lulu Sweigard, P.D. :

You can be either sitting or standing-

-Visualize your torso as a sandwich in a vertical position with a back layer, a middle layer of filling, and a front layer. Watch the back layer sliding downward.

– Simply focus on the image and your breath. Take time to notice anything you can about “the sandwich”. What type of bread, what is the color, the texture, what is the filling? Slowly watch the back layer of bread sliding downwards.

– After a moment or two, notice any changes that might have occurred in your back. You may even want to visualize sending a smile to your back muscles and thank them for all the work that they do in supporting you all day long.