Forget WD-40 for rust prevention. It evaporates.
I've had good luck with Breakfree CLP.
http://www.break-free.com/?location=/products/index.aspDeveloped for the Military as a "multi-purpose product to maintain their weaponry, it became known as the "impossible specification" because of its severe requirements. The first product to meet the challenge - Break-Free CLP."
Great for guns, tools, etc. Put a few drops on outside locks and they will work like new for about a year before needing more.