The original post was mrissa's observation that
Which is where it gets interesting.
Avoiding NyQuil is probably within the capacity of most writers. But do your archers fire arrows? Before guns? Does your hero have a strong suit? Before the invention of cheap paper, and so of playing cards, and so of bridge? Is something spartan in a world without Sparta? Is he an assassin in a world without the Assassins?
One trick that I found lies in your reading. I recommend reading lots and lots but for this trick, you have to read old books. Written in English, or translated into English so that the translation, not the work, is old.
You don't have to do anything special with them. Just read, and read, and read. Relentlessly. You're not trying to learn anything specific -- you're training your inner ear to recognize words as modern or of an older age. Plus turns of phrase, and metaphors. Or just getting experience with English that is not relentlessly modern. It does carry the danger of "speaking forsoothly" -- picking up archaic turns of phrase that don't help -- but it gives you a wider palette to pick from.
It helps to start with relatively recent works: move back from the early twentieth century into the Victorian, and earlier. This helps you pick up the vocabulary as you go, so you don't have to keep on going to the footnotes to figure out what Shakespeare was saying there.
Update: Humm. I forgot the warning. Okay, here's the warning: This will change the way you read forever.
Once you have trained your ear like this, you will not be able to turn it off just because you are reading someone else's works. This means you will be annoyed by other writers' tin-eared attempts to write about a culture that lack the concepts they are referring to.
The sacrifices we make for our art.