Even if we didn't have the science to back it up, we instinctively know that sitting all day is bad for your posture and back. We've already covered how stretching your wrists can prevent pain there, but reddit user thefalcone found this simple back stretch that can alleviate pain and tightness.
It's called the piriformis stretch, with the piriformis being the muscle around your spine, bottom and pelvis area. (I'm not a doctor.) It's quite easy to do and just requires you to cross your legs as shown above, keeping your back straight and leaning forward a bit. This is ultra important if you sit in a lousy office chair all day, and just takes a few minutes every hour or two to maintain your health.
Alternatively, you can approach your boss and say that if he wants to gain about 30 minutes of productivity time every single day, he should buy you a nice Herman Miller or Steelcase chair. That will easily pay off after a year or two.