I've tried every anti-fog product I could get my hands on and they all work to one degree or another.
I haven't found one that didn't degrade the quality of the image.
In particular, avoid the ones that are wax based.
The one product that I know of that I haven't tried is a product called spectramar-barris (sp?). Years ago when I was diving we used it to keep our diving masks from fogging on the inside. Maybe I'll find a dive shop and see if they still make it.