Soft T-shirt or glasses wiping cloth
Warm water, or a solution you can buy for the purpose. Or maybe sunglass cleaner solution.
Wipe straight out from the center of the disc, rotate a little, repeat until you've gone all the way around. Don't rub in a circular motion.
That's it.
EDIT: Or drop it into a saucepan full of boiling water for ten seconds, then dry. Seriously.