Don't use Rain-X; it will fog some face shields. Spitting in it like a dive mask doesn't work either.
Check with the motorcycle shops. There's a couple of products that work well. The Fog City Pro-Shield works very well, and never runs out. Fog Off! is a spray on product specifically for helmet shields. It needs to be re-applied periodically.
http://www.motorcycle-superstore.com...ro-Shield.aspx
http://www.whitehorsepress.com/produ...oducts_id=5867
I use this sun blocker strip on my motorcycle helmet. It works really well.
http://www.whitehorsepress.com/produ...oducts_id=3691