The advice I was given was to try and make the language neutral and avoid modern inflections, both in terms of word choice and grammar. And it really does work. If you read some of the big classics in epic fantasy (GRRM, Feist, even Tolkien) and historical fiction (McCullough, Renault, Follet), you'll see this is exactly what they do. The only deviation from this is to modify speech to show class, not least working or upper - which, of course, presumes the main characters are middle class and therefore speak reasonably properly.
A few examples:
Dated:
"Golly gosh! The Dark Lord has risen!" the elf shouted.
"Gee, whiz. Now ain't that a setback for us," the dwarf said.
The human rubbed his chin in thought. "Jeepers, that's some good timing. As we're here, we can show him what's what."
Modern:
"Hey! The Dark Lord's risen!" the elf shouted.
"That's a setback," the dwarf said.
The human rubbed his chin in thought. "Not a problem. We can kick his ass."
Neutral:
"Look! The Dark Lord is risen!" the elf shouted.
"That works against us," the dwarf said.
The human rubbed his chin in thought. "While we are here we may yet defeat him."
The examples are quick and rough, but the point should be clear - the more you utilize modern speech idioms, the quicker your writing will date. Even worse, modern usage can make the setting feel modern - which is fine when writing YA fiction (I've read books that do that) but the more neutral use won't clash with the setting, and can help your writing remain palatable for longer.
Hope that helps.