My complicated answer:
For me, it is of the utmost importance to nurture and protect the integrity of the setting above all else. That means no fantasy/Western crossovers, a lack of magical items and high-level characters, and most certainly no firearms, industrialization, or anything like that that might provoke an industrial revolution.
Unlike MoogleEmpMog, I am perfeclty happy remaining in the Tolkienesque "tropes"; I see no reason to add in more modern elements that I don't like, and that I would see as contaminating the setting. I myself believe there's plenty of life left in the classic plain-vanilla Tolkien-clone setting, and that you can just as easily put fresh spins on old ideas as create new ones. YMMV.
Anyway, there are several steps I've taken to protect my own version of the setting from contamination:
-First off, oil and gasoline are in far too limited a quantity to supply the needs of an industrial revolution, and they do not work the same way they do in real life-sure, oil can burn trolls, but it can't burn to power an engine. If we're going to be messing with ecology and physics, I see no reason why we can't muck with chemistry, too.
-Gnomes are the masters of technology. They can make primitive versions of the printing press, strange devices the likes of which you might see in the sketchbooks of Leonardo Da Vinci, and come up with all sorts of other labor-saving devices, but these things must all be powered by muscle power. It is also worth noting that humans cannot get gnomish devices to work for them. A human and a gnome each trying to build their own printing press, each following the exact same blueprints, will find that the gnome's printing press invariably works better than the human's, which is more likely to fall apart than anything else, although clocks and other devices could possibly work when built by humans.
-With no combustion, the only sources of power are water or wind, and usually far more common brute muscle power. Sophisticated things such as primitive cranes, rock drills, and other labor-saving devices based on counterweights, leverage principles, and other things, but they all have to be powered by hand. Wind and water power are usually insufficient for larger-scale devices, which means no clockwork-powered submarines or non-magical airships.
-The mortal races are still using the very same weapons and armor technology they were using five thousand years ago; and five thousand years hence, they will be wearing the same suits of plate mail and chainmail, and wielding the same swords and axes they do now, although obviously the styles and appearance will change with time.