This is a problem that has persistently irked me since the first time I was on an airsoft field.
I play at least 50% blind at any given time due to fogging of my goggles (and often get shot for it.
Thusly, in this thread, we shall post techniques on how to clear up this sort of thing. (See what I did there? "Clear up" Get it? Huh? Huh?)
I'd post some, but obviously, I don't know how.