Completely contary to the comments from some posters I always keep my helmet indoors above a radiator so that it is warm when I set out. This reduces the liklihood of misting up, after all, misting occurs when water vapour condenses on a cold surface so having the visor warm will help reduce this. Also, a Pinlock works extremely well at reducing fogging, specially in the wet. Again contary to another post, my main problem with fogging is my spectacles, not my visor. I use Fogtech DX on my glasses and this works well to prevent misting. If it's not actually raining, having the visor open a tiny bit will prevent fogging.