I guess it depends on what's wrong. I've had more back pain than you can shake a stick at, but what works for me might not work at all for you, or might even make it worse. So take it with a grain of salt. But for me, things that help (in no particular order) are:
* Bar hangs (just grab a chin up bar or branch and just hang and let gravity pull things back into line. 30 seconds at a time, or a minute if you're a masochist).
* Back stretches (cat/cow seems especially helpful).
* Hip stretches, particularly piriformis stretches.
* Twists with a broomstick
* Walking. As strange as it sounds, adding a bit of weight via a vest or backpack actually seems to help more than just plain walking.
Those too!