One thing that I never mention in my stories is how the characters are dressed. Well, except when they are being described for the first time, when the main character first meets them. How important is it for the reader to know this information? Do famous authors spend time doing this, say, mentioning how the characters are dressed on different occasions?
If how the character is dressed is important to the story, then you should certainly describe it. If not, then don't bring it up. Give as much detail as is relevant.
If, for example, you picture Mr Jones as always being immaculately dressed in a formal business suit and carrying a walking stick, I'd mention that, at least once up front. Or at the other extreme, if you picture him attending a very formal event, like a wedding or a reception for the governor, wearing beat-up blue jeans and a cowboy hat, I'd mention that. But if you picture him wearing a business suit to his job as a lawyer, or wearing blue jeans when he's working in the yard, well, that's pretty much what the reader would expect, so it probably doesn't need to be mentioned.
Or if an important point of the scene is that the heroine is looking incredibly beautiful and wildly sexy, going on at length about how just what and how much she is showing could help to build the mood. Make clear exactly why all the men can't take their eyes off of her, etc.
Even if how the person is dressed is relevant, you can often say all the reader needs to know in a couple of words. Like, "Sally was modestly dressed and carrying a briefcase ...", or "Bob arrived looking more formal than was really appropriate to the occasion ..." might be quite sufficient. The exact color and style of their clothes might make no difference.
I certainly would not go into a detailed description of what every character is wearing under normal circumstances. This would really bog down a story. Like, "As Smith, the master spy, entered his hotel room, suddenly three assassins leaped up and attacked him! The first was wearing a blue shirt and black pants, with a black belt. His shirt had a pocket on the front. He carried a white handkerchief in his back pocket. His belt had four holes and a slightly triangular shape to the tip. His shoes were made of brown leather with rubber soles. One of his shoelaces had a damaged end. There was a ridge pattern to the soles. The second was wearing a brown shirt and khaki pants. The pants were originally permanent press but the seam was somewhat faded from many washings ..." Unless any of this later proved to be important to the story, that would be really tedious, and would really take away from the potential excitement of an action scene.
A follow-up thought: If how a character is dressed is relevant to the story, you should convey this information as early in the scene as possible. If you do not reveal this early on, then when you DO reveal it, the reader has to backtrack in how he pictured the scene. Like is you say, "Sally arrived at the pig farm early in the morning and set to work cleaning out the manure from the pig stalls ...", the reader is going to immediately have an impression in his head of Sally wearing some grubby clothes suitable to such work. If five pages later you mention that Sally is wearing a formal ball gown which she has carefully managed to keep immaculately clean through this task, the reader is going to have to rethink his entire mental picture of the scene up to this point. That kind of backtracking is often the punch line of a joke, and it might work for a humor story, but anywhere else it would just be disconcerting and confusing.
Like most things, it really depends on what kind of story you're writing and what effect you want to achieve. I can think of an example on both sides of the metaphorical clothing descriptor coin.
There's a technique present in older fiction where the author will lengthily go through the description of a character's outfits, gardens, or houses in order to express the personality of the character or the nature of the setting as told through objects and people. Parts of Chapter 2 and Chapter 7 of The Three Musketeers by Alexandre Dumas are good examples of this. Applying this type of long, seemingly inane description to somebody's outfit at various points in the story can serve a few purposes-- expressing the personality of the character with the way they dress, expressing their role in the setting (Do they stand out? Do they look like the average person? What does this say about the character?), and even delving into their backstory (Are they wearing some kind of antique, maybe?). This can all be done without the character saying a single word, but you run the risk of being long-winded and dull. There's a reason many modern novels have shied away from this approach.
Then there's the Chekhov's gun approach. Or, I guess in this case, the Chekhov's socks approach. If you have a description somewhere in your work that Chekhov's socks are neon blue with backlighting, then those neon blue backlit socks had better alert the cops during his climactic midnight chase scene with their outrageous luminosity. There's no reason to describe "her brown hair with a fresh flower in it" or "his cleanly hemmed trousers" because that isn't going to save them from the nuclear blast later on in the story. This is a really minimalist approach. Let the readers think what they want until you mention otherwise.
I personally like it when authors use a combination of both. I'm thinking of Atonement right now (I don't have the book on me) when Ian McEwan describes Cecilia's dinner party dress. If I remember correctly, the description doesn't have a point besides for essentially saying "Cecilia is like, really hot right now and totally knows it, guys, like, daaaang." But, hey, it works and it gives you a visual image for why Robbie is interested in her. And again, if I remember correctly, the book doesn't have much else describing things clothing-wise, because there wouldn't have been a true effective purpose behind those kinds of descriptions in the rest of the novel.
So, in conclusion, describing a character's clothing is only as important as the purpose you give it.
Above all, you build an impression.
Your character is a rich cloud of emotions, feelings, attractions, fears, repulsions, desires, cold logic and secrets. This is what really acts upon the reader and interacts with their imagination, "makes the story tick". It all boils down to emotional level, and it's the only level that truly matters.
Wouldn't it be nice if we could mix these like chemicals in a test cylinder to produce just the right mix? But as writers we must use more indirect approach. Especially, "show, don't tell." So you can't tell the reader your character is organized, or rebellious, or sinister, or kind. You must show them. In order to do so, you reach to standard tools in writer's arsenal: events, descriptions, little and big actions.
Clothes can tell a lot about certain traits of character.
Think of a man wearing a suit. Impeccably white, perfectly tailored? Or salmon-colored with overstuffed shoulder pads? Or one grey, clean, ironed, but decades out of fashion, faded and nearly threadbare on the sleeves? One ten numbers too big, and with just bare chest underneath? Shiny golden with pearl patterns? With elaborately mismatched sides, one sleeve white, one black, but the composition not clashing?
That's all some ways to show certain traits of character, be it permanent, or temporary quirks of mood. Say, shirt partially sticking out of trousers, one side of the collar sticking up? Certainly the person didn't pay much attention wearing them, be it to laziness, or to being in a hurry.
Don't think about describing clothes in terms of some duty, to fill some gap. Think it as a neat tool of building character. If you can describe someone's character in a way that entirely skirts the issue of their outer appearance, that's great! You managed not to bore the reader with unnecessary descriptions! Our protagonists tracked effects and actions without ever seeing the character, and now as they learn about his/her name, the readers know the character so thoroughly they just don't need the looks of their clothes.
But more often than not you are forced to introduce the character's first impression in span of half a page or so, and giving them outer looks, including clothes, is the easy, fast, reliable, tested and true method of providing that first impression. It's a very basic tool, and it's not that you "should describe clothes". You should create a compelling, complete first impression that won't leave the reader puzzled and grasping at straws as to who is given character. Giving the description of clothes is a good way to achieve that.
In my experience, lack of description leads to a boring story, while over-describing gets nauseous very quickly. (In regards to character clothing)
So I developed a personal style that I think you might want to consider using. That is:
For example, let's say Mr. John was wearing a black shirt, brown jeans, a blue coat and a cowboy hat. (That would be the general description that comes first) this description doesn't change (Or at least, it shouldn't) and should be given the moment the character is introduced.
Then, depending on how your plot moves, a detail description can be given. An important note should be made that this detail description should not be about his clothes (That is to say: NOT about anything previously described) it should instead be something like: Mr. Jack noticed that Mr. John had a strange looking antique wristwatch or a tattoo of a puppy or some other accessory.
At last, we go on to the gradual description. That is in regards to his clothes. For example, Mr. Jack said, "That's an ugly coat you gots on yar'self John, why haven't you been wearing dat worn thin' since last year?" to which John replied, "It may have a fews holes, but it's still a good coat.".
In so doing you would have managed to say that John is wearing a worn blue coat that has holes and that he has been wearing for a few years, though John still thinks it is a good coat.
My point is if you cover the most important descriptions first and add a few more details later, the later ones help define your characters clothes better. I think that suits my style since I like my characters to have fairly well-described clothes.
Clothes could be important if the social standing of the character is relevant to the plot. Take this description of the heroine from my screenplay:
"She dressed in Prada, her handbag is Coco Chanel, and her shoes were made by Jimmy Choo. She’s used to “the best.”
The woman is one of few female partners in a professional firm in the late 20th century, and the story is a demonstration of the Mexican proverb: "The rich also cry." (Los ricos lloran tambien.)
And if this is important to the "setting," it's best to introduce it early.
The answer is (as almost all answers relating to this type of question) it depends. There is no set way to write a story. For me, in all cases of description, the issues are relevance, POV, and 'real time'.
The question being . . . "What would the character notice?"
e.g. "Lucy followed the maid through the large library to the drawing room." - There are very few rooms than you can cross in 5 seconds. Other than the library being 'large' the character has no time to make observations.
"Lucy followed the maid into the library. "Take a seat." she gestured. "Mr Smith will be down presently."" - During the wait the character has time to observe and describe the entire library.
The other issue pertains to a writer's method.
"How do I look?" said Mom, clutching her purse. "You'll do," I replied, feigning disinterest. In truth I was a little shocked. It had been three years since Dad died. At some point in time she was entitled to move on with her life. Bob Tucker had invited her out on a date - and she'd accepted. It hit me - this woman did not exist purely to be my mom. She was a woman. She scrubbed up well. Any red-blooded man would want to hit that. "Make sure you're home by eleven," I added.
Here I'm looking for an 'empathy fill'. Rather than impose my view on the reader. I want the reader to recall an image of somebody who was shocked when confronted by a woman who they viewed as nothing more than a sister, a mother, an auntie - who shocked them when donning heels and make-up. Going into a detailed description may ruined for them. Letting them fill in the details themselves let's them believe - I was there. I know that woman.