Here's a video I put together covering a Myofascial Release technique of a hip flexor to help with lower back pain.
Giving the lower back relief is a complex process of understanding how the body works as a global structure, however it is important to start at an isolated point before where the root of disfunction begins before address the body as a whole.
There are few places better to start for lower back pain than the hip flexor complex.
Stretch intentionall, not habitually,
Naudi