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Exercise. If you have a weak back, first go to see a physician. He can recommend some light exercises to strengthen muscles on your back.

I always suggest swimming, it involves all muscles in your body and is a good way to build up your body. Especially if you're not a gym type.

But all in all - exercise, going for a walk, try to reduce amount of time in a chair and/or replace some other "leisure" activities for a good old exercise.