While I'm preparing for a con this weekend, I was going through my gear and thought I would mention something that I use in my helmet. I don't use fans in my helmet - it's normally not too bad here for exterior troops, but interior troops can get muggy. This causes my visor to obviously fog up.
A fellow trooper used this in my helmet for the last convention and it worked flawlessly. The product is called 'Cat Crap'...yes, the most amazing name for a product ever. It comes in a cream or a spray. The cream will last you a lot longer, I would think. Just apply some (you don't need much) and wipe it off with a rag. Works perfectly!
For Canadian Fetts, you can get this at Marks Work Warehouse near the counter for the same price. If you try to get it from the States, I've seen shipping be 3x the cost of the product.