If you have an LED or LCD screen that resembles the inside of a pizza box, it is time to clean, but fear not. You've probably already got what you need to restore some dignity back to that display.
Start with a dry microfiber cloth to gently wipe away those loose crumbs and crud.
If you have some more stubborn grime like that sticky dried cola, moisten it with distilled water, or distilled water and white vinegar mixed in equal parts. You'll want to shut off and remove the power source.
DON'T use soap or cleaners containing alcohol. If you don't know what's in it, then steer clear.
Be prepared to swat away the hands of those compulsive screen-touchers.