I never understood the appeal of trekking poles until I started carrying 80+ lbs packs down into the bottom of the Grand Canyon for a living. Now, I wish I had started using them sooner. Perhaps my knees would be in better shape.
Trekking poles, used properly, can take countless TONS of cumulative weight off your knees and lower back by shifting that weight (and impact) to your arms. They excel at ascents and descents (and their benefits are more noticeable) but can also add some benefit on relatively level walking.
There are lots of guides to tell you the "proper" way to use them, but in my experience, every person uses them a little differently. But, in general: planting your right pole in front of you as you step your left leg forward while consciously engaging your arm to take some weight off your lower body as you step your right leg forward (while planting your left pole). Follow through with the stroke, and you can even push yourself forward a bit before bringing your pole back to the front.
It takes some getting used to, and to incorporate it into your stride.
As for other gear / techniques to help with back (and other) pain:
They say a properly fitted pack should put about 70% of the weight on a comfortable, well-fitted hip belt, with 30% on the shoulders. (I personally prefer about 95 - 100% on my hips, using my shoulder straps to just keep the pack balanced upright, with no downward weight.)
Think about your stride. Do you heel strike? Try landing more on the balls of your feet to reduce jarring impact with each step. (And to build some awesome calves like tree burls.) Good practice: walk barefoot on hard floors / cement (without weight -- or with, you choose) and see how quickly your heels start to complain.
Yoga -- both as a regular part of your life, and while on the trail. A few sun salutations in the morning goes a long way toward limbering up the spine for a day of hiking.
I'm a big fan of Superfeet(tm) insoles. A bit spendy - but make a considerable, long-lasting difference in foot comfort (and thus leg, knee, and back comfort). I treat myself to a new pair every season. Money well spent.**
**I am not in any way associated with, or compensated by Superfeet for saying this.