Former trainer, BS in Kinesiology chiming in here:
Most guys go at strengthening their backs entirely wrong.
Here are some things that you can do that don't directly involve your erector spinae (the muscles you're likely straining) that will decrease the incidence in back injuries.
1) lose weight. If you're packing around even 10-15 extra pounds, that puts undo stress on your lower back, changes your center of gravity, and adds premature wear and tear. Adult men should have visible abs, no excuses. You accomplish this with diet.
2) strengthen your glutes and hamstrings! The correlation between back injuries and weak posterior chain leg muscles is eye opening. RDLs, back squats, back extensions, and nordic hamstring curls should be your bread and butter. If you can't currently do a nordic hamstring curl with your own bodyweight...I guarantee you that your back pain will disappear when you are able to complete just 1 un-assisted nordic curl. Dead serious.
3) strengthen your anterior core musculature. planks, front and side, hanging knee tucks, etc...are all money in the bank for bullet proofing your back. Ditch the situps, they suck. Think about your core musculature as your backs bodyguard. Most guys train abs by moving them, you need to train your core to resist movement.
4) work on your mobility and power. most dudes are stiff and slow. If you can train your muscles through a FULL range of motion, and work on force production through explosive reps on compound movements and plyometrics, the incidence of injury decreases exponentially.
One last kicker that the great Dr. Stu McGill teaches (literally the Godfather of spinal health and rehab) is that walking is your best friend. We are built from the ground up to walk, and our low back is the anchor that every piece of us is tethered back to. If you make a point to get a brisk 30 min walk in everyday, you WILL see a difference (oh and it's really good for your brain).