My goggles used to completely fog over in hot weather or whenever I wore a dust mask. Then a motorcyclist friend told me about a spray he uses on his face shield. Now, I simply treat my lenses with antifogging spray (see Source, below). Temperature and humidity determine how long each application works, so you'll have to experiment a little. The first time you use it, apply two coats. After that, spray on a fresh coat at the first sign of fog. A 2-oz. spray bottle contains about 150 applications.
KleerVu, (877) 329-6450, www.kleervu.com
KleerVu 2-oz. spray cleaner, $7 including shipping.