It's tough to prevent VF fogging in some conditions, but it's relatively easy to get rid of it, at least temporarily.
Suck, don't blow.
I'm serious. If you draw air rapidly past the eyepiece by sucking wind with your lips really close to the finder, you can evaporate the condensation. Blowing on it, of course, only makes it worse.
Movie cameras have electrically heated viewfinders.