That's the condensation-caused fogging I guess? Now days you can opt for an anti-fogging permanent film for your high-end glasses (at least I recall such a technology was invented around 2010 already). Also I saw some DIY solutions a while ago as well, being researching the ways to prevent diving mask fogging, similar solutions were applicable for an ordinary glasses (since then I've got this mask: http://www.tribord.co.uk/easybreath-surface-snorkelling-mask-blue-id_8304664 where it's been resolved in even better way)

Hmm,... perhaps, that mask might help to figure a simple anti-fogging technique for glass wearers - you can try to form an air-tight "half-mask" under your glasses and over the nose/mouth with your palms, and then inhale, so the colder air "wash" the lenses from inside coming from the top and sides - that's exactly what that snorkeling mask is doing, but it's also diverting the exhaled air downward and out to the water... In fact I'm using a very similar technique with my telescope eyepieces for ages: if fogged, I'm simply surrounding the eye side lens with my hands, leaving a small gap between fingers for air intake right over it, and inhale through the top of that tube several times. The warmth of your hands also helps to prevent repeating fogging for a little while.

Back to the subject, in my at the office GHB I'm keeping a pair of swimming goggles - for a haz-mat/smoke protection improvising (plus, we have the Ocean withing the walking reach). However for a glasses wearer you might need to invest into some dioptric inserts to use them comfortably. On other hand, that would make an indestructible spare pair of glasses for you as well. If I would wear glasses, I'd just scavenge some old pair for lenses and make such an inserts myself with the Dremel and some epoxy glue. Also you can order such swimming glasses having your glasses prescription on hands (just google "prescription swim goggles"; avoid expensive options, as you need a pair just for emergencies, not for an everyday use, so cheaper options, without the astigmatism correction and other precise features, would suffice).