Back extensions or "hyperextensions", bodyweight only. They have been the cure to my back issues and now I only get back pain after a long day or two in a row of jackhammering.
When I started after my last back "tweak" I could only do 2 sets of 8 (9 reps would trigger a massive spasm), now just over a year later I regularly do 2-4 sets of 25 (4 sets if only doing core once a week, 2 sets if doing core multiple times per week, depending what my splits are), about a 3 count to round out and touch the floor with my fingertips, 1 count to come up to a slightly hyperextended position.
I also do a hanging "toe touch" stretch at the end of most workouts to specifically loosen up my lower back.
In university, one of the things I recalled from my qualitative ergonomics class was that lower back muscular endurance and flexibility were both negatively correlated to back pain, so when I started to have severe back pain I took those two factoids to heart and implemented them into my routine.
Also correlated with back health is the ability to do a front plank for two minutes, so as a long term goal shoot for three minutes.
Side plank was also correlated with back health, so shoot for the same (again, as a long-term goal), 2-3 minutes.
It's not entirely scientific as it's based entirely on correlation, but the statistics say that if you have great muscular endurance in lumbar extension and the front and side plank, as well as at least average lumbar flexion flexibility, you have a very low probability of experiencing severe back pain.
PS - I assume you already know, but it is worth noting that it is also extremely important to keep your quads and hip flexors flexible in order to maintain proper pelvic tilt (and thus lower back health).