As an office worker there's little I do everyday besides work at my computer. If only I could sit comfortably I'd be much happier with the job. There are enough causes of back pain that what works for one person might not for the next, but here are some of the things I've done which help me. If they don't help you, just remember to keep on experimenting. Every 5-10% percent improvement is worth it!
1) mkenzie back roles help a lot. Almost any chair is made better by adding one. I've also found that paper towel roles can be useful, as they compress less than the mkenzie rolls.
2) Some amount of reclining can help. The worst possible thing is to sit in a chair that forces you to slump forward.
3) getting your feet under you can help. IE if you have a chair without arms you can sit with your legs wide and your feet directly below your rear, pushing lightly on the ground. I find spending at least some of my sitting time this way helps. It's especially helpful if you have to sit in a bucket seat.
4) standing some of the time really helps. I have an elevated desk (about 4 feet off the floor) and this allows me to stand for 30 minutes at a time. It took a long time to adjust to this, but it's clear that it helps a lot to not sit all the time. On the other hand, it's critical to not stand all day. Esp. later in the day I need to mix standing with sitting.
5) trying the chair out before buying is pretty important. Just because it's expensive doesn't mean they made it lower back pain friendly! After many years of looking for a better chair I've come to the conclusion that a $10 yoga ball from walmart is the best option for me.