Silica gel, or any similar product is the answer. I spent approximately 2
years in Bolivia (late 1969 - early 1971) with a $19 pair of Swift 7 x 35
Payless specials. I spent the better part of a year in the Chapare region of
the Cochabamba province (humid all the time and usually hot). When I wasn't
using the binos I kept them in their case with several packets of silica gel.
They never fogged up or grew any funny looking green stuff!

Dan Heyerly
Eugene, OR
danheyerly at msn.com