I'm sure many digital artists (as well as other professionals regularly working while sitting) experience lower back pain, or at least stiffness.
For the two years I've been doing this full time, I've found two things that help me deal with pain and stiff muscles.
One's hot baths. Of course, you wouldn't have those every night after a long day of painting, but I find that once a week is just fine.
The other is a magical artefact called the Iplikator.
I lied, there's nothing magical about it. You'll definitely run into all kinds of yoga or acupuncture mumbo jumbo on the internet. All it really does is poke your back, make the bloodflow a bit better and relax your back muscles.
And that's enough. If it's placebo effect, doesn't matter, it works great. It didn't improve my eyesight, it didn't improve my posture (much), it doesn't cure any diseases.
I use it almost daily, 20-30 minutes in the evening, laid on a hard surface (soft or springy surface makes it less painful at first, but also defeats the purpose).
Some people wear a t-shirt, I don't. It's less effective than the naked back.
No need to wriggle or do yoga on it, I just lay down, it hurts a bit, but then it gets very comfortable. I watch a movie, or an episode of a tv show (just enough time). Be careful not to fall asleep though, getting up after a nap hurts!
A side effect that I noticed (which might be just me, or placebo) is that I sleep much better after I've used the mat.
I've had my mat for quite a few years, we bought the original version based on the Russian prototype, for some 20 USD back then. These days it's a bit more expensive, but 25 EUR or an equivalent in dollars will pay for itself very quickly. (I recommend getting the big one, D1 or just the full size that covers your entire back)
Note: I'm not paid by makers or resellers of this thing. ;) Just sharing something others might find useful.
Also, this is in no way a substitute for exercise and other aspects of a healthy lifestyle. I'm not a doctor and don't have any medical training, so don't take my word for medical advice.