I usually sit at a desk all day and all evening, and find that these things help:

• Good chair. Height adjusted for my keyboard/mouse height. Upright back. Lumbar support. Comfortable-but-supportive seat.
• Good posture (when I remember to pay attention to it).
• Split keyboard. Mechanical switches that don’t require too much pressure.
• Good display. IPS panel. Brightness turned down much lower than the default. Calibrated, optionally to a slightly warm color temperature.
• Muted room lighting. Nothing behind me bright enough to reflect much on the screen.
• Comfortable clothes.
• Cup of water. Regular trips to the kitchen to keep it filled.
• Frequent short breaks. Start the laundry. Get a snack. Look at objects outside. Wash a dish. Bring in the mail. Make the bed.
• Exercise. At least 10 minutes daily; preferably 30 minutes or more. Stretches. Squats. Rhythm games that require movement.