If you wax your glasses before you go out it helps the spray roll off the lens and not turn into a salty spot.

A couple times a year I polish my sunglasses with carnuba wax, dont use more that a spec of it or you wont be able to get them clear. I also carry a small lens cloth in a zip lock bag that I can clean and polish my sunglasses between races. I usually just use spit to polish the lens. For some reason spit also helps with fogging - especially if you do it before you go out when the lens are dry and you let the lens dry after you spit polish the inside.