Keep the computer screen at an arm’s length away
Take breaks (to go get a glass of water, bother a sibling/friend, 10 jumping jacks, etc) every 20-30 minutes.
If you’re uncomfortable, move! It’s good for you to change how you are sitting frequently.
Make sure that your forearms and wrists are in line, and keep your arms close to your body. This will keep you from experiencing discomfort in your hands and wrists.