Lower back: Lie down and extend your arms out to the side ("T" shape).
Roll your right leg over your body to the left, keeping your right hand on the ground as best you can and try to touch your right knee to the ground on the left side of your body. Roll left leg over to the right, keeping left hand down.
Upper back: requires helper that can lift you, and reach around you.
Stand straight and cross your arms in front of your chest, making an "X", hands to opposite shoulders. Have your helper stand behind you, reach around with both arms in a bear hug. Then they pick you up. Causes your spine to 'stretch' & arch backward a bit, I guess.
Gets great cracks from midway on up. If your helper is not tall enough,
have them stand on the first step of a staircase.
Upper back / no helper. Sit on the floor, legs together. Begin to lean back as if to lie down. Pause when the middle of your back is touching the floor; kind of 1/2 way into a crunch. Then just drop the rest of the way to the floor.