Control yourself! After about 15 playthroughs, I've finally paid enough attention to note my weapons deteriorating after an enemy dies. During their death animation, when they release the blue stuff and evaporate/turn into ragdolls, hitting them severely reduces your weapon's durability. Go ahead and try it. You can see the red durability bar shorten with each swing, much more than it would with a normal hit on an enemy.
So, in summation: don't swing your weapons more than you have to. I think certain enemies reduce it a lot when doing this; like the exploding mummies (because they weren't annoying enough).
edit: slight revision! It's not the ghostly stuff that reduces the durability; it's hitting corpses at all; period. Why? Because Dark Souls.