Anyone familiar with masks will know that the first thing you do after buying a mask is to rub non-gel toothpaste on the inside and outside of the lenses. The reason is that the manufacturers apply a coating to the lenses to protect them and make them look clean and bright. Somehow, the toothpaste treatment does not always do a good job of getting the protective coatings off.
Before every dive, one should treat your mask in some way to prevent fogging. Commercial defog products like 500 PSI are probably the most popular with divers. The defog products work well and are easy to use and apply. Some divers use baby shampoo as a defog agent. The baby shampoo seems to work just as well. Yet another group of divers use plain old saliva. Some of the veteran divers say that nothing works as well as saliva.
Recently I read an interesting article about a nanotech coating that will permanently prevent fogging on glass surfaces. A scientist at M.I.T. who helped develop the coating, said in the article that the coating will be inexpensive and will never have to be reapplied.
If this technology ever becomes commercially available, you might never see people spitting into their masks again.