A lot of this is still personal, but I think most of it could apply to a lot of descriptive language in fantasy, in one way or another.
1) Archaic words shouldn’t be used only in descriptive scenes. If you write the whole fantasy in an archaic flavor, like Dunsany or Eddings, then words like “ichor,” “eldritch,” “chalcedony,” and “avaunt” may fit right in. (Well, not ichor for me, but then I agree with Ursula K. LeGuin that that word is “the infallible touchstone of the seventh-rate.” Her essay on fantasy style called “From Elfland to Poughkeepsie” is invaluable. Read it). However, if most of the fantasy is more towards the style of Mercedes Lackey or Marion Zimmer Bradley, these kinds of words won’t fit nearly as well. It will also be very noticeable if you only use them when you’re trying to be “impressive,” such as when describing the entrance of a king.
To quote Ursula K. LeGuin again, a lot of fantasy writers know instinctively that they want to give a higher and more formal flavor to the story. The mistake is to think that they can do that just by injecting certain words. Descriptive language should not be a purple flower planted in a pink garden unless you’re trying to make a point, such as parodying the faux-Elfland style. Descriptive language should flow seamlessly with the rest of the story.
And this is the second point:
2) This can be done without seeming purple. Compare these two descriptions:
The palace was the most beautiful thing Kyshar had ever seen in his life. White towers rose towards the sky like needles that would pierce the sharpest dragon wing, glittering as if made of chalcedony, reflecting the pink and purple hues of dawn back in a dazzling array. Shadows still lay on the gardens, not yet lifted by sunrise, spread in a chiaroscuro of infinite complexity. Kyshar could still see the flowers peering through, however, slight flashes of saffron and turquoise and rich radiant purple that promised beauty as rare as dragons when the shadows were lifted. The tower walls curved like the wings of seabirds sweeping in from the ocean, and the call to prayer that echoed from them was as haunting as the cries of those birds as they came back from the land of the sunrise.
Kyshar had never seen a palace; he supposed this was what one might look like. Sharp white towers stood at the four corners, flashing back the sunlight. The walls in between curved like birds’ wings. The way to it looked open, though, marked by paths leading through thick-flowered gardens.
The first description has far too many adjectives, similes, obscure words (like “chiaroscuro”), and sentences piled on top of each other haphazardly. It’s as much a description of the gardens as the palace, even though the first sentence announced it was the palace Kyshar would be concentrating on. The second one may not praise the surroundings quite so lavishly, but I chose what I thought were the important details—especially cutting back on the gardens, since this paragraph focused on the palace—and one simile I liked.
The first scene makes me puke in disgust; I think the second would be workable. And it’s especially important in this case, because the character whose viewpoint I’m writing from would be unlikely to notice as many little details as the first passage gives.
Which leads to yet another point.
3) If you’re not writing from an omniscient point of view, the descriptive language should fit in with your character’s personality. I’ll go with the example above. Kyshar has never seen a palace before, as the first sentence of that second description announces. For that matter, he’s never seen many people before, and is terrified that most of those he encounters will send him back to his brother, whom he just escaped. He would not turn into a bard singing the praises of the palace, or mentioning seabirds when he’s never seen the sea.
I think the only characters who can reliably get away with a lot of what fantasy authors pile into descriptive passages are bards, poets, and others used to noticing a lot of physical detail about everything and fashioning the words to describe it. The taciturn warrior whose life is fighting would probably notice first whether a place could be well-defended, and only last or not at all how pretty it looked. The person who’s lived in the mountains all her life is much less likely to gawk around wide-eyed than someone who’s never been there.
The usual trick for getting around this is to give a description of the place as from outside the character’s view and then move back into their head, or into their head for the first time. I think this is cheating, particularly when the descriptive parts of the book are the only ones written that way. Remember that you have time to build up your setting in a novel, and that not every detail may be important anyway.
Especially not every little detail about your character.
4) Cut down the amount of purple prose spent on your heroes. I put this in a different category than just simple description, although I still find the seemingly obligatory paragraph about the protagonist’s eyes, hair, upturned nose, and weapons boring. This includes using different adjectives and similes constantly for the character’s looks, noting whenever the color or expression in the eyes changes, having every other viewpoint character reflect on the hero’s beauty or handsomeness, and noting in contexts where the writer should be concentrating on something else (like battle) just what the character looks like. I’d also put identifying characters solely in terms of looks, like “the green-eyed woman,” in this category. (This is something that drives me nuts with a lot of fanfiction).
An example of all of these:
Calling the character’s eyes “green” on page 3, “viridian” on page 20, and “verdant” on page 25.
Having your hero, in a conversation with the heroine, pay such close attention that he notices when her eyes “darken” with anger, “swirl” with thought, and “brighten” with excitement—and having this happen in every single conversation.
Having four viewpoint characters beside your heroine, Pinderella, and making every one of them just about faint with Pinderella’s beauty and give an extensive description of her.
Writing about a dying enemy noting the bright flush in Pinderella’s cheeks and how nice she’s looking today.
This is something I have a lot of fun with, but then, I write a lot of parodic fantasy. If you’re not, keep an eye on it. You’d be amazed at how silly it sounds.
5) Avoid clichés in your similes and metaphors. “Swift as the wind” and its kind are obvious, but people still use them. There are also others that have become exclusively attached to one kind of description, such as “Her eyes flamed.” Have you ever seen this in real life? It sounds stupid, and conjures up the picture of fires actually burning in the back of the character’s eyes, when usually the author just wants to convey anger. Why not “She gave him a hard stare,” or something else that could imply more than just one emotion? The worst clichés are just another way of telling rather than showing, since they hurry the reader along one road of perception (and sometimes an extraordinarily silly one).
6) If you do make exceptions for these, make sure it’s clear why you’re doing it. Perhaps your spoiled princess notices every detail about the peasant’s hovel because it’s so much dirtier than anywhere she’s ever been. Then it might be proper to emphasize the difference, and in so doing say something about your character. But if she’s used to palaces, then describing every single nook and cranny and treasure of the elven palace is a bit silly.
Similarly, I think it’s all right to try to convey a sense of awe with descriptive language, such as trying to emphasize the grandeur and glory of Faerie. But the descriptive language shouldn’t be the only way you know how to convey awe, and it should be used judiciously. Don’t have the character (unless a bard or poet or someone with a similar excuse) react that way to mundane places, and practice other kinds of writing, such as hard and precise, in places where you don’t have time to practice the awe-inspiring things, such as when writing about sword-fights.
It’s a shame that even fantasy’s special gifts, more formal words and descriptive language, often don’t get used in moderation.