A cheap and fast solution is using saliva.
I use quite frequently in all my glasses (swimmingaswimming and spearfishing) since ever. It is quite effective.
Water the lens. Spit on the inside and rub it. Wash it.
For spearfishing there are also some film one might use on the lenses (they solved the fog problem in one of my masks) :
A more expensive solution, that I know of, is athletes that in every competition wear a new set of goggles.