Don't slouch on the couch with the laptop. Limit non-essential computer use. Stretch your arms, hands, neck, and back during breaks. Breaks should be taken every 30-45 minutes for at least 5 minutes.

Dr. Taraneh Razavi (seen here sporting a GMail T-Shirt) is the official Google Physician. She shares tips and links to some good software utilities that help you prevent repetitive stress injuries at work:

That goes in my essential reading list. And don't forget to blink!