variables include your rig and climate you're diving in......
I used to have problems with fogging, now I don't. My solution:
When diving in Caribbean waters, I install my camera (oly c4040) in an air conditioned (dehumidified) room, into an Ikelite housing. I include three, (2 gram each) "silica gel dessicant packs" along with one feminine napkin.
http://www.preservesmart.com/products.htm#...caGelDesiccants
no affiliation
Try to keep the silica gel packs positioned away from direct contact with the most likely source of most of the heat (imho, the batteries). Otherwise you may be reintroducing the moisture collected right back into the enclosed environment.
I may be overprotected now, but the system works for me.....
hth,
b