I bought a new helmet which fits a lot more snugly around my chin and cheeks, and I found I was having misting issues, even in the summer, so Itried a couple of things which I thought I could share, especially since it's winter for you northern dwellers.
The 1st was the Rain-X product which comes in the black bottle, the product designed for use on the interior of auto glass - it worked very effectively, and I'll continue with it.
The other strategy was to buy a visor insert, the type that creates a double-layered space on the visor. This double-glazing effect prevents misting, and it seems to be working really well. I've had experience with these before, and although we aren't in the depths of our winter, I have to say I'm impressed so far.
The brand I ended up with is an Italian one called Raleri. I found it here:
Raleri LightShade Anti-Fog visor insert/Clear | eBay
It was by far the cheapest option, for those of us stuck in a very non-competitive and small marketplace, and seems to be working out well. Free shipping, prompt delivery, genuine article from Italy, all good.
As a side note, the helmet vendor offered me the same kind of product made by Pinlock, using a peg-assisted mounting system already fitted to the visor. Local counter price - NZ$75 (=US$60 approx). Price for the same item in the UK - £16 (=US$24). Yee-ouch.