Preventing fog inside snow goggles
When you do sports, your body needs to cool down. This is done by sweating: Your body vaporises water to cool itself down. Since your brain need a lot of blood, a "huge" percentage (something like 50%) of the heat is emitted by your head. If you wear the googles on your head, you put them into a "humid environment". Therefore, as soon as the temperature drops, they can not maintain the vaporised water. The water condenses.
Possible Solutions (Based on experience and my reasoning):
Buy goggles with at least three layers. I had both two and three layer goggles. This makes a big difference. I presume the outer layers help to keep the inner layers warmer? Hence the inner layers are less likely to condensate water vapor.
Most Goggles have a vent at the top and at the bottom. This helps to keep the air moving, letting warmer air with water vapor to escape through the top.
The same issue occurs with swimming goggles. I'm not sure 1 and 2 will work in the water version, but some swimmers spit into their goggles. This is not a particularly good idea for snow goggles. I would not recommend. This works for swimming googles because of the impurities in the saliva. Which will simply mean that you need to clean your goggles soon after. Anyway the whole process will most likely result in you scratching up your goggles.
I presume some goggles may be made of or coated in a substance which decreases this effect. It is debatable how long this effect will last.
Store your goggles inside your coat where they stay warm. While they are warm they are less likely to condensate water vapor when you first put them on. I have seen other people do this, but I can't remember being forced to doing this myself.
Beware of balaclavas, neck/scarfs and coat hoods. These have a tendency to either obstruct the vents or otherwise redirect a lot of water vapor from your mouth and nose into the goggles. Where this has happened to me in the past it can be quite difficult to work out that it's the water vapor from your lungs as opposed to the skin around the eyes.
This guide looks quite comprehensive: SkiTripGuide