What I do is I place a thick book on the floor, lay down face up on top of the book…I try laying on top of the book so that my upper back is on top of it. Then, slowly, I do a crunch and exhale at the same time do so. Then, I move the book to a new position with respect to my back and repeat....
And now...sorry but
Disclaimer: If you try this, you do so at your own risk and you take full responsibility for doing so. Also, this describes what I do and NOT what you should do. Whether you imitate me or not is your choice.