How to Keep Your Mask From Fogging

Why masks fog
Masks fog when moisture from your face evaporates into the space between your mask and then condensates on your lens. This typically occurs at two times: when you sweat a lot and release lots of moisture from your face or when your face is considerably warmer than the outside air. The first situation typically occurs during hot days while the second, similar to a windshield that fogs, happens when the outside air is considerably colder than the air around your face.
Anti-Fog Spray
Many companies (paintball companies and other companies) market anti-fog sprays that are designed to keep moisture from condensing on flat surfaces. The basic principal is to spray a mist of the anti-fog onto your lenses and vapor will no longer gather on your mask and fog it. People have reported mixed results, but it is the cheapest and easiest way to stop fog. I have found it to be helpful in delaying fog, but on a hot day it doesn't seem to prevent all fogging.
Mask Fan
Some masks come with built in defogging fans while some masks can be upgraded to use fans later on. Fans sit above the goggles and blow a stream of air over the goggles to cause condensed moisture to evaporate, thus eliminating any fogging, similar to a defroster on a car. Fans are somewhat expensive, require extra batteries, make a considerable amount of noise and are prone to break, but they do effectively reduce fog, provided they are working correctly.
Thermal Lenses
Thermal lenses consist of two lenses with a thin air-filled space between them. The air between the two lenses acts as a barrier between the air near your face and the outside temperature. This protective barrier keeps the inner lens closer to the temperature of your face, which limits the rate that moisture will condensate on your lens. Thermal lenses come with or are an optional upgrade for all but the most basic masks and I have found them to be the most consistently effective way of reducing fog.
Conclusion
Some people (usually those who perspire readily) have foggy masks no matter what they do while others never have to worry about fog. Any (or a combination) of the above methods can help keep your mask from fogging - experiment and figure out what works for you.