Monkey business? Like special gas additives or something like that? I've heard of that trick for my '84, blocking that hose to the ignition computer, but I've heard that it's only necessary if you fail NOx, and my '84 isn't even close to needing that trick (my '93s don't have that vacuum hose, or that computer).

All I've ever done on all my cars is use synthetic oil (AmsOil), and premium gas (almost always Exxon). Close to inspection time, I put in a bottle of Techron (hopefully cleaning the injectors) and follow with an oil change. And I've blocked off my preheat orifice to the air filter box (and discarded the hose -- the inspectors have NEVER lifted the hood, and I doubt they'd know it's gone even if they did).
Can't think of anything else that I've done to the engine.