Exercises for lower back pain?

I have a weak back from carrying a cello around a lot. I haven't carried it much recently, but my back seems to have gone again, three days before I'm due to start at music college. I have had Alexander Technique lessons in the past, and will take them on again at college. Please don't tell me to see a doctor, as I fully intend to visit one when I move cities in a few days. But I don;t want to turn up with my back as bad as it is - so does anybody know of any exercises/sites that give exercises to alleviate it a bit?
This works for me...
Stand with your back against the wall. Slide down a little, and move your feet so that your back is against the wall and your knees are bent at about a 45 degree angle. Tilt your pelvis upward so that you are pressing the small of your back against the wall. Hold this position for about 5 seconds. Repeat 10 times.
I do this two or three times a day and it helps a lot. It increases strength in the muscles supporting your back and core so that your back/spine doesn't have to do all the work. And surprisingly, sometimes it offers instant relief too.
Good luck to you.

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