Out-of-Whack Back?  See if this works. 

1.  Use a yoga mat or blanket.  Stretch it out on the floor.  No pillows.
2.  Sit on the mat and ease your back slowly down to the floor.  Rest your head on the mat.
3.  Bend your knees up with your feet flat on the floor.  Keep them there for the duration of the exercise.  Rest your arms along your sides.
4.  At first, your back may not feel like it "fits" the floor.  Just remain in this position for a while -- 5 minutes maybe.  Gradually, you will relax and your body becomes more comfortable.
5.  When you're ready, pull your heels close in to your buttocks.  Push into the floor at your lower back and begin to lift your buttocks up.
6.  At the same time and moving slowly, feel your back come up off the floor, one vertebra at a time.  Keep raising your buttocks while also feeling each vertebra rise off the floor.
7.  When you get to the neck, hold.  Your back should be off the floor and your stomach and thighs as far off the floor as you can get them.  Feel the pull in your upper leg muscles.
8.  Slowly, begin to lower your back, one vertebra at a time.  Bring your back down to the original position.  Relax.
9.  Follow these same instructions, completing 3-5 times, raising and then lowering your back.

I've observed this exercise as a part of a yoga lesson.  I use it every several days.  When my back gets tired, or feels like it's out-of-whack, just going through this exercise helps eliminate feelings of back discomfort.

In addition, I periodically take a nap flat on the floor.  The position is the same as for this exercise only my knees are resting against each other (a pillow can be placed under the knees also).  Whatever crookedness seems to be developing in my posture as I age, this exercise and nap help to straighten me out.  I'm reminded to walk head up, shoulders back and belly tucked in.  Can't beat that!