Sit on a chair at a desk. This is especially important if using a laptop for longer than 30 minutes.
Keep a good posture. Adjust the chair and laptop for a "neutral" posture. This means ankles, knees, hips and elbows are at about 90-degree angles and hands are in line with wrists.
Relax arms, neck and shoulders. Most muscle strain centres on arms, neck and shoulders so try to keep these relaxed. Typing and using the mouse should be light, and hands and arms rested when not typing.
Don't sit too close to the screen. Sit about arm's length from the screen, depending on individual eye conditions.
Take regular breaks. Take five minutes out of every 30 minutes to rest both eyes and muscles. Stand and walk or change position to do other things like reading. Look at an object about 10 metres away for 20 seconds.
Make sure there's enough light. Work where lighting is sufficient and make sure your screen is free from glare.