I'm pretty lucky in that I have a fairly large corner office. In that office, I have a foam roller, a stick, and various balls of different sizes. If stuff acts up, I shut the door and hump all the above. I also have a few kettlebells, an axe handle (for swings for my goofy sport), yoga block, and a swiss ball.
Much of the advice that has been previously covered are essential and I do them daily, when I'm in the office: drink lots of water, pace while dictating, stand while talking on the phone, and do speaker phone conferences while pacing.
One thing that I've found with my chair is as Yoon mentioned is sit it on it's lowest possible setting, then seriously doing a "low rider" lean back to take all stress off my back when I'm forced to sit for prolonged periods (writing briefs/reports/memos/etc).
It's doable. Just gotta find what works for you.
All the best,
Lifting heavy stuff is fun and relative......