Desk job - back and belly pain - HELP!

Leslie
Any of you working mamas have any tips for how to not be completely miserable at work?
I have a desk job so I'm sitting for 8 hours a day and my back and belly are killing me!
I'm 30 weeks. My doctor said to just drink a lot of water because the pain is due to stretching but I notice it is a lot worse when I am sitting like this.
Help!