I don't know where are you from.
Here in my country (Poland), there is are labor laws that guarantee 5 minute break from starring into display for every 1 hour of work with computer. They are hidden somewhere in work safety laws.
I'm trying to make the best use of them, get up from my chair, prepare something to drink, walk to server room to change backup tapes, or bring some order back to my desk... There is "Workrave" software on my workstation which reminds me about breaks I'm entitled to. My superior used to think that my behavior was a little eccentric, but quickly found out that my efficiency does not suffer from it.