A couple of weeks go my back got a little stiff and sore. Nothing unusual in that, I've suffered two significant back injuries in the past and at 6'3" I'm on the tall side on usual! Anyway, when my lower back gets tight I usually get down on the floor and do a series of stretches.
I work systematically through the hamstrings, quads, gluteus and piriformis. That usually does the job, but not every time and not this time. The one muscle that I often forget, and I'm not sure why, is TFL. Perhaps it's because I haven't yet found an easy way to stretch it on my own, although I've seen a few possible ways of doing it.
My solution was to get out one of the spikey massage balls I bought a while ago. Lying on my side, I placed the ball under my side and into TFL. Carefully I moved up and down and round in small circles over the ball. It's quite difficult to control the depth, but after a while I found it relatively easy to support myself enough to control it.
You could probably do a similar thing with a tennis ball or even the two tennis balls taped together tool I made. For me it solved by lower back pain and freed up my back considerably.
If you're going to try this on yourself and aren't sure where TFL is located, the easiest way to find it is probably to lie on your back and find the bony bit of your pelvis that sticks out on the side. TFL starts just behind that bump on the outside and runs down to the outside of your thigh. If you put your fingers on your thigh up near your hip and then turn your foot inwards you should feel a muscle tighten under your fingers. That's TFL.