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I found the following in an Amazon review.  If the glasses fog, we tend not to use them:

"You can make ANY goggles virtually "fog free" (under most conditions) by marking them with plain bar-soap. Simply make a few swipes on the inside of the goggles with a plain white soap bar, then wipe the marks around until the goggles are clear and clean. Cotton cloth works best. Paper towels are OK, but they will eventually scratch the lens. Use light pressure. You do not need water.



Details about this method...

Do not use old-fashioned hand soap that was made with Lye... it burns your eyes!

Do not use fancy expensive soap that has scents or colors... many (including myself) find the colored soap makes rainbows of prismatic streaks, and that the scented stuff ruins your taste-buds when lunch time rolls around.

Just use PLAIN WHITE SOAP bars... Johnson's baby soap, Ivory or cheap generic stuff from any American soap maker... Avoid the foreign stuff! Other countries have lower sanitation standards, especially for products that are not meant to be used internally."

http://www.amazon.com/Dewalt-DPG82-11C-Concealer-Anti-Fog-Safety/dp/B000RKQ1NI/ref=sr_1_1?s=hi&ie=UTF8&qid=1318599569&sr=1-1