Well, for starters, you need to realize that the body isn't supposed to sit all day... spinal structures inevitably make adaptive changes to compensate for this behavior, and that's what ultimately leads to chronic back pain.
That being said, not everyone has the luxury of changing jobs... so make sure your work environment is ergonomically sound.
From there, you might consider performing the posterior chain exercises discussed in a book titled "Foundation" by Dr. Eric Goodman. Here's a link to the book on Amazon (it's about $14). In my opinion, it's very useful in combating the effects of prolonged sitting.
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