I often sit at a desk all day and all evening. I 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, tented keyboard. Mechanical switches that don’t require too much pressure.
Good display. IPS panel. Light anti-glare surface. Backlight that actually dims the light source, either without pulse-width modulation, or with PWM at such high frequency that it cannot induce flicker fatigue. Brightness turned down much lower than the default. Calibrated at that brightness setting, 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 full-body movement.
I often sit at a desk all day and all evening. I find that these things help: