WD-40 is a water displacer (and it was the 40th formulation tried, hence the name). It is not a rust preventative, and on top of that it gums up over time.
I used to use it for rust prevention, but no more. I would use it if I went hunting and the gun got dunked in the water, and all I had to use was a can of WD-40 in the trunk - but I'd degrease and re-rustproof the gun immediately after getting home.
Thanks to those who've posted on Renaissance Wax and on Blue Wonder's Armadillo. I got a tiny can of RenWax from Milt Sparks when I sent my holster for repair (thanks, guys), and the stuff is great on leather - no prints at all, and it has a very lusterous shine. Now I think that I'm going to put it on my carry gun and some other metal that I want protected. If the museums use that stuff for high-carbon steel blades and suits of armor, it must be good.