I'm very fond of elegant mechanics in RPGs; systems and tricks that are novel, simple and utterly functional. My favorite RPG of all time, Greg Stolze and John Tynes' Unknown Armies, is full of these wonderful little bits of mechanical inspiration. Flip-flopping percentile dice, the Hardened and Failed notches in the Madness Meters, the different levels of skill checks, Focus Shifts... the list goes on and on. With that in mind, here are a couple of mechanical solutions that I would love to integrate into my eventual Wild West retro-clone (if their authors are willing, that is.)
-Robertson Games' Aiming and Evasion rules: Very much like Unknown Armies' Focus Shifts, this is probably the most straightforward and still mechanically satisfying way to cover active evasion, without bogging down into minute bonuses and penalties. They would work perfectly in a Wild West firefight setting.
-Tales of the Rambling Bumblers' Super Simple Combat Maneuvers: Now this is what I'm talking about. Taking the finicky nature of 'what penalties do I have if I want to disarm him?' and reducing it to its utterly simple core. If you hit, you disarm him, unless he wants to hang on to his sword so much that he risks his life and takes the hit. I'd go beyond just my Western game and suggest that anyone who's looking to add a little 'rulings, not rules' to their Classic D&D game should definitely check these out.