If I have a really stiff back, I tend to lie down a couch or equivalent, then lie down sideways, usually to the left side, and if so use my left arm to push the lower back in while I rotate the torso the other way.
A favorite way is to just be standing up, then spread the feet about 2 feet directly apart, and swing a roundhouse like arm movement with the fist one way so the torso swivels, letting the arm pull the torso around to the side. I'll also often do this by pushing in at the small of the back with the other hand while doing it.
I also crack it just by leaning backwards so I'm bent over backwards at at least a 90 degree angle. I've even had my hands touch the floor before. I'll even lean straight backwards like that sometimes and then after a second or so let an arm swing over strong across the body so it effectively turns the back left/right while bent over backwards, getting several cracks. I don't do this too often and have to be careful and deliberate when doing it. Basically this is the same as the above, but while bent over backwards.
I can also crack it just by sitting on a chair, scooting forward in the seat, and pushing in at the small of the back with the left hand to push the midsection forward.
In my life, I can't remember ever having a back injury, and I think exercises like this have contributed to it.