I found this one out in Jamaica and live by it now everytime I go diving and snorkling.
Baby shampoo. yup, you read right. Get a small bottle of baby shampoo ( I use the shoppers generic brand in the travel sized bottle) squirt a bit into your mask, coat it well, then rinse it out just before you hit the water. Since I've started using this, I've never had my mask fog up. Ask FlopNFly, she's a satisfied customer. :D
I've shown it to quite a few people now who all gave the same feed back. Holy @$## it works! :eek: (last time being one of the dive Masters in Cuba who was skeptical but is now a convert)
Give it a try.