I didn't realise this, but apparently after many long hours of use the inside of a monitor's cathode tube can accrue debris attracted by it's electrical field, making the resolution a little murky and the brightness not so bright anymore. This free online program cleans that particulate matter off for you, from the inside out. It takes a little while to load, but I saw a surprising improvement after it had completed.
Monitor Cleaning Program
It might take a little while to load, but it's worth it.