Another really simple thing you can do is walk around while you're thinking. I know you probably incorporate this into "stand up when not operating the computer," but it bears repeating and emphasizing. Not only does it get you away from your keyboard and chair, it also helps stimulate the thought process.
When I'm on the phone with clients, I will stand/walk around my basement for two reasons; it gives me a break from sitting and I can talk loud without interrupting others who are working or kids who may be napping. I have a computer in the basement where I can browse whilst on the phone, with no chair so I don't get sucked into anything. (P.S. I'm typing this standing up)
A few years back I had forearm/wrist issues on my mouse hand. Some simply adjustments in my posture (Hint: don't waste money on an expensive chair) was enough to stop the pain. The icing on the cake was working for myself. Instead of 8 solid hours in front of the PC clicking around, I now have 100 things to do in the day, only 25 of which may be actual development. The end result is more time away from the machine and less pain which means a potentially longer career.
