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.
Cheers.