Take a break!
When I'm coding I usually get so involved that I don't think to stop and take a break. This usally means that when I have a good day of coding, I end up with a nice backache. However, there is a cool utility called
Workrave that will remind you to take breaks (30 seconds every 5 minutes) and even walks you through some exercises during the long break (10 minutes for each hour). I've just started using it and it seems to work pretty well.