Avoiding Back Pain

You could stand, lay down flat on your back or on your side like the Buddha. Maybe a monk wouldn't do it but sitting on a Zafu in a recliner is nice on the back. Laying down and sitting in recliners can lead to falling aslee-p more often but one can sort of keep a mindful eye on the first signs of of falling asleep and conquer falling asleep. I know you can practice mindfulness in any position. Have you tried walking meditation? One pointed meditation obviously needs a steady and stable posture.

You need to check if the pain is caused by (sore) muscles or spine. If this is a muscle problem then doing some back excercises should help in two or three weeks time. Also you may try some streching after meditation to relieve muscle tension if there is any.
If this is a spine problem then you need to check with a doctor because that may be sign of some deeper problems.

Where the pain is happening is really important in determining how to adjust your sitting posture. If the pain is in your lower back, believe it or not, you are probably fine. That is an indication of poor muscular strength which will gradually improve as you practice. Judging by how your pain creeps up on you afterwards, this sounds like it might be what's going on. What you are feeling may be delayed onset muscle soreness. It's no different than what a bodybuilder might feel after squatting heavy. The next day, walking gets tricky!

If the pain is not in your lower back, but rather in your middle and upper back, your issues are probably posture related. Most people rightly try to maintain an upright sitting position. What they may be doing may even look perfectly fine. Nevertheless, there can sometimes be a subtle curving of the spine that will result in pain that radiates throughout the middle of the back or shoulders. The best advice I ever got on how to fix this was to lean back ever so slightly. Don't go overboard, obviously. You should go back just until you begin to feel your chin lift. This is one of the easiest ways to ensure that your shoulders are aligned over your hips and that everything is coming down in a straight line and into the cushion.

Based on your list of details, I can share some of your experiences. I meditate in Burmese position, I get pain in my mid-to-lower back, and I also have poor posture in general. So here are some suggestions:

Hard Carpet

My preference is to meditate on a hard carpet and you might also find it beneficial. Cushions and thick carpets can both throw off our balance and cause us to over-exert the back or some other set of muscles. And on the other hand, meditation on a hardwood or worse, concrete floor presents its own set of problems.

Next, we have a pretty standard approach to back pain:

Apply the meditation practice to the back pain.

This is sort of like powering through it as you suggested, but it's powering through it with the right set of tools. You don't want to make yourself miserable.

For example: Usually pain arises first; then we identify with the pain; disliking arises; then we identify with that; thoughts, narratives, stories arise; and it keeps going on and on. The untrained mind creates suffering out of a natural phenomenon.

So try to catch it at the very beginning: Observe the pain as pain. Should that fail and you move on to disliking, observe the disliking. If you move on to anything else, observe that, ad infinitum.

While this doesn't seem to address the problem of how to avoid back pain, it actually does in two ways:

1. Like Bakmoon said, this help your back muscles to strengthen over time. During the sitting posture you are breaking down the muscles the same way an athlete strengthens his or her whole body; and when you sleep the muscles rebuild and come back stronger than before. And like Bakmoon says, it will probably take a month or even several months.

2. Your mind will learn to stop making it worse. By seeing the back pain just as it is (nothing more, nothing less), your aversion and thus your desire to avoid the pain will decrease. When your mind is not making the problem worse, the body will have more resources it can devote to healing and general upkeep. And then, if your back really needs something, you will probably see that more clearly too.

I would also suggest to try sitting for longer periods of time, like 15 minutes, or even 20, 25, 30 if your mind can tolerate it. 10 minutes at a time may not be enough for your back muscles, or for your mind, to really get the point.

Don't take my word for it anyway, as I'm still learning and we all have to figure this out for ourselves. Hopefully this is helpful.