For example (and you're probably not making this mistake) Do not for the love of things that are good and holy in this world make your characters talk Elizabethan.
Nothing pisses me off more than people assuming nay insisting that a 12th even a 16th century Norman or for that matter that a 16th century Welsh, Scot, Pict, Angle, Briton, Cornish or Saxon would sound like Shakespeare. Chaucer would be a much closer approximation but even he was writing in the educated english of the day, not colloquial english/scottish/welsish/cornish.
For a good YAish take on on fantasy diaglog I personally think the Dragonlance novels are a decent example as are the Forgotten Realms stuff though RA Salvatore could get pretty freaky with the affected speech.
Though is YA more that Heinlein's juvenille's? If so I'd look to Brian Jacques's Redwall series.
Depending on the setting, you can add little bits to the language to indicate that it's not-quite-modern-English, which will (hopefully) set up in your readers' minds the foundational premise that "this is not modern", and then you won't have to push too hard to make your point.
You don't mention the social class of the characters you're writing about, and that will make a huge difference, because of levels of education and background, but you can do most of the work simply by using some mechanical tricks. You can also set the scene by being careful what your characters talk about.
For instance, sugar used to be something that was a rare thing for anything other than the richest of families to have on a regular basis. If you find other things that in this world are taken for granted, but in your world are not, you can shift the reader's viewpoint that way, and then you don't have to worry so much about language at all. (Dragonlance did this, for instance, by making steel rarer and thus more valuable in the post-Cataclysm world than gold).
Modern: "Jenny, mom says I have to go to the store and buy her a five-pound bag of sugar. She's making cookies!"
Reworked: "Dacie, mother asked me to take the pony to Coopersmith, and buy a packet of sugar. She is making a treat for your birthing-day!"
In the second one, note the use of "mother" instead of "mom"; this slight formalism can indicate a more formal relationship between children and parents. "Taking the pony to Coopersmith" shows that a trip to the store is not just a spur-of-the-moment thing, as "going to the store" is, for us. "Packet of sugar" says, even though we're making a whole special trip to Coopersmith, we can only afford a "packet" of sugar, thus indicating its relative rarity in our setting. And "treat for your birthing-day" sets up the idea that treats are not something that happen often, and require a lot of preparation. This ties together the pony trip to another town, and the special purchase of a mere packet of sugar. And yet, in this whole sentence, the only word I used that is not one in common modern English is "birthing-day".
-G
If you have your characters use words of one syllable as much as possible when they speak, that might help.
What is most important to me is to give each character a distinctive voice. On a surface level, maybe one is upper class and uses bigger words, one a farm boy and uses earthy language, one sarcastic, one shy. While you should always indicate who is speaking if there is the least chance of doubt -- that is: John said, Jane said -- it is always nice if the reader can tell who is speaking from the voice alone.
If you are very, very good, you can have the characters talk the way people did at that time. The best example of that I know is the novel True Grit. But this is an advanced technique that I suspect is very hard to pull off. I've never dared attempt it, though I have borrowed "voices" from my childhood, especially Cajun.
I'd avoid slang unless (1) you're writing something set in a specific historical period and can use something from it, or (2) can come up with something new.
By-in-large, I dislike reading stuff heavy with archaic formal speech. A little to emphasize a formal or solemn occasion is okay, but I like plain, modern, slang-free dialogue, i.e., nothing that overtly dates it--I don't want to read about a group of people in a high fantasy setting having a "blast" during an evening of "clubbing" at the local ale-houses. Also, the narrative and the POV characters speech should match pretty close, assuming a fairly limited 3rd or 1st person POV.
A simple rephrasing of such idioms might help distance the dialogue from the modern day. This might be as simple as providing the meaning behind the phrase instead of the phrase itself ("Well, people will remember that bit of foolishness for the rest of my life") or it could be a bit more complex, like creating your own phrases. Such a phrase, however, would need to be intuitive enough that the reader can still comprehend it easily ("Well, mountains will forget that before you'll let me."
"Yo' Mama," Joe said, spoilin' for a fight.
I am very curious about whether this works or doesn't work.
Gut feel is that it would normally work smoother in first person than third, but I think it would definitely work (and [probably has) in third. You just have to make sure the characters "voice" isn't a distraction. It seems you would also want to keep the POV close/limited most of the time (everything told only as the character sees it). When you do have to get more distant, it might be necessary to retreat to a more neutral narration.
quote:
I am very curious about whether this works or doesn't work.
I believe it can work, Rick. I've seen something like it once or twice, and I think OSC did it a little in Magic Street at times. Of course, success depends on the author's skill and practice will help. It may not be perfect the first time, but don't give up on it.