I've been going to Alexander Technique for quite a few weeks and although my arms are getting better very slowly, the effect on my back has been astounding. Rather than anything done in the lesson, the real difference has come from practising what's called the "semi-supine position" for 20 minutes each day.
Here a couple of websites which explain it:
Semi-supine1Semi-supine2 (note that despite what the picture shows, your hands should rest on your stomach to help your shoulders relax)
A lot of my back muscles which were 'permanently' tensed up have loosened amazingly, meaning I've gained a lot more freedom of movement in my upper body and have no more pain. It's also straightening my spine out, which helps me to sit up straight. According to my Alexander teacher, there was no way I could physically have been able to adopt the correct posture a month ago because my spine was curved in the wrong way.
I was a bit sceptical at first, but the difference it made within a week was simply amazing and I totally recommend to anyone who has back or shoulder pain to do this. It's very relaxing too and helps me de-stress and clear my mind before I go to bed.
Also, see if you can get hold of a posture wedge to sit on when you play piano. I've tried out someone else's and they really make it impossible to slouch! Another good thing to do is to make sure you get up every 10-15 minutes when you practise and have a walk round. Do a few stretches to loosen up your back and shoulders. This will help prevent hurting your back from sitting in a static position for too long. Here are some yoga stretches I've found on the internet and are very helpful:
Yoga stretchesSorry this post has been so long - hopefully this will be helpful in some way...
Hope you get better!
Rosie