The way I would set expectations:

Make allusions to mysterious "ancients", along with a creation myth that foggily relates a golden age when civilization was much, much more advanced than the present, followed by some sort of downfall.

IIRC this was one of the few things that Sword of Shannara did sort-of right. Other inspirations to check out would be A Canticle for Leibowitz, Nausicaa of the Valley of the Wind. Game-wise, this sort of thing appears in Harn and The Fantasy Trip (though in both those cases, the ancients are dimension-travelling beings who brought tech & magic together).

The language that you use to describe the downfall should convey what you're after without hammering it over the players' heads. E.g. in Nausicaa the cataclysm is referred to as the "Seven Days of Fire". Your description of the ancient civilization should sound like a cargo-cult description of industrial technology, e.g., "Even common people travelled by means of giant mechanical dragons which were built by the wizards of the age." This'll get the message across while being thoroughly in-setting and therefore immersive.

Gene Wolfe's Book of the New Sun is also a great example of taking SF tropes and talking about them in-character as fantasy. Two great details that establish the ancientness of the setting: the ground is so full of artifacts that no delver does not find at least one in every spadeful of earth; all the mountains have been carved into Mt. Rushmores.

Quote from: Tavis;473035Gene Wolfe's Book of the New Sun is also a great example of taking SF tropes and talking about them in-character as fantasy. Two great details that establish the ancientness of the setting: the ground is so full of artifacts that no delver does not find at least one in every spadeful of earth; all the mountains have been carved into Mt. Rushmores.

Ok, tavis, this is one I agree with.  I look forward to reading your stuff, though I always withhold disapproval.  The book of the new sun is gaming gold.   I have a collegium tortoris in celtricia...

It's not SF level but a real world example of living in the shadow of a greater age is medieval Rome.  At points, it had 1/20 of the population of Rome during the empire, at it's height it had 1/5.  It was separated by a thousand years of culture change.  Pastures and farms had monuments and structures of unknown origins; they almost always had origin stories for these things but they were almost always wrong.  The very rich sliver of Romans had near modern lifestyles, with fancy heating systems, running water, and a transportation infrastructure that brought in things from all over the empire.  

To the dark age and medieval Rome, things like the aquaducts and the coliseum must have seemed made by giants.  Who knows what strange pagan rituals the temple had been used for?