If you are in the US, there is a brand of product for car widows called Fog-X made by, approprietely enough, the makers of Rain-X. Works like a charm.

You cna also by something in a push tube at Ski Shops that is to keep ski goggles from fogging. it's a waxy substance but it works.

Taking a bar of soap and scribbling on the mirror like a crayon and then buffing that out with a towel works well enough in our house.