I'm a huge fan of Lifehacker. They keep their content fresh, and it's always interesting and helpful content. They recently mentioned a program called AntiRSI - it's basically an egg timer on your computer that tells you when you need to take a break. It doesn't let you cheat, either - if you move your mouse or type at all, it will reset the timer until the Mac is idle for the given time. After using it for a week, I can honestly say that I'm more focused, comfortable, and energized when I'm at my computer now. I highly suggest the download if you're a Mac user (if you're not - seriously, get with the program! :-) ).
It's important to take regular breaks when you're a heavy computer user. This program will help you stay disciplined. Don't neglect your health or your eyes - download this program (link below).