I'll throw one out there. Aerospace 303 Protectant is absolutely the best stuff I've found, but it's kinda pricey. I've used several brands/types over the past 30+ years (but not all brands mentioned in this thread), and this stuff works very well, does a good job in repelling dust, and lasts for a month or longer.
http://www.303products.com/shop303/index.cfm?category=58