Spill the beans. My back pain is the result of limited activity that keeps all of the muscles associated with the hips and back tightened and not relaxed.
I think you pretty much hit the nail on the head. Once you have an old back injury, from then on it's tight muscles that cause the pain.
Here's my system, which is based on working a desk job. I'm 31, weigh 210 lbs, and I had a discectomy on my L5-S1 disc in 2010. They had to remove nearly half the disc, which had broken off and was impinging on my sciatic nerve. I was given the option of steel rods, but I chose not to get them in hopes that rehab and PT would get me back to normal. The first year of recovery was a bitch, worst year of my life by far. Since then I usually have a relapse once or twice a year, but lately I've been on a pretty good streak of being pain free.
Sitting vs. standing - Sitting all day with good posture shortens your hip flexors, which makes it hard to stand up straight. Add bad posture (slouching) on top of that, and your discs compress in the front and open up in the back. Your core muscles that are supposed to keep your spine neutral also become weak or out of balance. Standing is great, but do it too much and you'll end up in pain when your muscles get tight. I have an adjustable desk at work, and I've found that not sitting or standing for more than 2 hours straight gives the best results.
Walking - Walk to the farthest bathroom in your building. Drink extra water so you have to get up more often. Park in the no ding section of parking lots and walk a bit more. Add a quick 5-10 min walk after you eat lunch.
Warm ups vs Stretching - Never stretch before a workout or stretch just for the heck of it. Do warm ups instead. 5 minutes on the elliptical is what I do at the gym. After the workout I stretch for a good 15 mins, but I don't try to force anything. Nothing worse than trying to reach that extra distance to touch your toes and then pull your back in the process. I mostly focus on my hamstrings, calves, and hip flexors. Don't do any weird yoga stuff where you arch your back. A neutral lower spine is your friend.
Lifting weights - Here's where I might lose people because you need a gym membership or your own weights. This has been huge for me though. My two favorite lifts are the hex bar deadlift (google it) or weighted lunges (holding a dumbbell in each hand). You can keep your lower back in a neutral position for both of those lifts. Squat and regular barbell deadlift cause me pain, because I have trouble keeping my spine neutral. Not many gyms have hex bars unfortunately. Obviously, don't overdo it or try to go up in weight too quickly.
Running - No need to go crazy. A 3 mile run once or twice a week is plenty. When I'm busy I try to do one gym workout and one 3 mile run per week. Warm up before or start out slow, save all the stretching until afterward. Run with good form, land on your forefoot, not your heel. I don't like to listen to music, because I don't want to zone out when I run. I try to stay focused on what I'm doing and not let my form get sloppy as I get tired. Walk the rest of the way if you feel any pain.
Activity vs. Rest - I try not to do physical activities two days in a row. Back pain sometimes doesn't show up until a full day after I do something, so I space out my activities to make sure I'm not making something worse.
Bed Firmness - I don't think it matters much. When I had my back surgery I bought a memory foam mattress, but recently I switched to a firm standard mattress and I think it's actually an improvement. I would say lean towards the firm side.
Stress / Outlook on life - I've read opinions that the majority of back pain is in people's heads, but I don't buy it. It's physical and it's real, but that doesn't mean that stress or negative thinking can't make it worse. Something to think about.
Meds - Stay away from opiates. The risk of addiction is too high. I don't keep anything over the counter on hand because I'd rather address whatever is causing the pain, but when I go on road trips I usually pick up a small bottle of Ibuprofen to take with me. Tylenol only deals with the sensation of pain, Ibuprofen actually reduces inflammation. Alcohol can be a surprisingly effective pain killer, but impaired judgement and not being able to feel pain temporarily means you might do something stupid that makes you hurt more the next day.
I think I've typed out enough, haha. I know everyone is different, but maybe something in there will be helpful.
