1. Look away from the computer and focus on a distant object every 20 minutes
You can strain your eyes looking at a computer for hours, let your eyes adjust and focus on something further away from you. Using a computer although out school college and so on has taken its toll on my eyesight. i have a "stigmatism" in both eyes from using computers and now wear glasses when using them. I wish i had known this tip throughout education.
2. Stand up
Every 30 minute or so stand up and maybe walk to your bathroom or up a flight of stairs and then go back to your computer. Its good just to get the blood flowing for a minute so your muscles don't get stiff.
Stretch your arms shoulders neck and even twist your body from just sitting in your chair you could youtube stretches to do whilst at your desk. Here's a quick video with three stretches you could try.
4. Adjust your monitor height
if your monitor is to low this can cause you to hunch or tilt your head down and can result in back or neck pain/stiffness (i also suffer from this) apparently if your screen is too high you can get dry eye syndrome so having the correct height of the monitor is essential.
5. Download flux