Woah I like the concept 
I like to look at specific designers and specific celebrities for inspiration of how my characters would dress--they're people that already made their own look, they kinda do a lot of my work for me. So I have a main character for my comic, Gordy, who wears a jumpsuit with this fancy jacket because he's a free hippie spirit (with a lot of money) who idolizes the boho-hippie style of the Olsen twins (who have a lot of money). So I didn't steal directly, but I took heavy inspiration from two of their looks and made it work for my style in the comic.
I have another character who's hair is based on Zooey Deschenel, she wears a 2010's indie throwback poncho because she's wearing second hand clothes that happened to be available at this school for plot reasons. It's got the 2010's silhouette of skinny jeans on the bottom and way too much knitting on top (complete with knitted fingerless gloves). It's also kind of a joke that she's stuck in this poncho throughout the comic, and honestly it's fun to draw because it is constantly flowing in the wind.
I got another who just dresses in a sweater-vest like it's a magic school because he's the closest I got to a magic school student, so he actually follows the dress code. Easiest person to dress.
But other than personality, I think the biggest thing is asking what your favorite thing is to draw. I love swooshy materials that flow in the wind, I love tailored pieces--those things end up in my character designs a lot.
My characters don't change clothes that often but I try to get into their shoes a bit and think of the style of clothes they want to wear. Then, I look on Pinterest for those kinds of clothes lol. I think it really helps to have some consistent style or motif like @magnavarra does.
My character's clothings are stricly connected to the general aesthetics of the story I am making. For instance, if I chose to do the worldbuilding around Middle Eastern designs, as it is with Menmar, my characters will dress accordingly, which means I'll do tons of research on patterns, moulds, dressing manners for men and women alike (in case there are gender-addressed dressing code, which is the case in such societies) and I'll play around with those concepts.
I'll post some examples here, as I did for the main cast a few months ago in order to promote my story:
Esther and Yami are a married couple in my series, and since they are both quite gender-conforming in their views and dressing code, I think I can illustrate the point I am making when being strictly faithful to Persian designs (I believe I mixed Safavid Era and another one as well, but I'll have to check my research references back again to be sure).
With Nadhine and Raheem I began to allow myself to play with the fashion concepts I studied, giving some sort of androgynous looks in some propositions, whereas I paid tribute to some formal/ regal designs in them as well.
TL;DR: for me, it vastly depends on the main aesthetics of your script/ story setting =)
Consistent colour palette and general silhouette is what I learned to use and love. So usually after I know the world the characters belong to and what their occupation is (student, traveller, warrior), I draw them in one outfit and choose some colours.
After that if I need to change the clothes, I decide what they will or will not wear, if they have a vast wardrobe, if they choose the clothes themselves, what color or item they would never wear, do they like patterns or not, etc.
Look at clothes from Riverdale for example, you always can predict what characters will wear, because they have a color palette and particular clothes and jewelry. So you just have the same things looking slightly different.
You can always have one character that changes clothes more and other who barely do this, because people don't always wear something new, many don't have more than 3-5 items of shoes or bags, most people don't have more than (roughly) 20 items of clothes they wear regularly 
I feel you, OP. I chose to give my characters changing outfits to make it clearer when it's a different day from the previous episode, and also so I wouldn't get stuck drawing the same thing forever, but it does slow me down every time I have to pick a new set of outfits. Luckily I like fashion, but then sometimes I get stuck drawing a tricky jacket or something for like 5 episodes, and that sucks.
Each of my characters has their own style, which makes it easier. One of them is a kind of emo dude who basically wears a uniform of a faded black t-shirt and black jeans, though I'll have to give him a jacket as the story/year progresses. For the others, I tend to use google image search for inspiration -- for instance one character is kind of mod, so I just search "mod fashion" and get some ideas from what comes up.
You're welcome! I hope it doesn't sound complicated, because it's really not 
there's that thing called "capsule wardrobe", if you really need changing their clothes a lot, you can use what you already have, add some items (don't put many items, like 4 tops and 4 bottoms are more than enough) and base their looks on this whole idea.
Uh...for mine I just base it off my sense of style. It's non-existent and boils down to...do they look like they'd be in a Metal band? Check? Ok. 
I never been good at fashion so I've been practicing it through my story on my friends recommendation. I use my friends mix and match method. I look outfits and draw the parts I like and combine them together. As for coloring, I usually decide a color scheme for the outfit. I try not to use too many colors in an outfit and I try to choose colors that would complement their eyes or hair, though sometimes I choose a color and let the whole outfit revolve around that color. That said, I do have standard outfits which are the outfits my character started in and I toss it in every now and then
Before I started my comic, I made a Pinterest board for the fashion each character wears, so I always have inspiration and reference and a personal guideline for your the characters dress. And even though it's all fairly casual fashion, each character has a distinct style and certain items of clothing or accessories that only that character will wear.
So the main character Skyler will wear straight-leg jeans (usually with rips or patches) and red Converse sneakers. He doesn't wear plain t-shirts, his shirts are all either Henleys or ringers or baseball tees, and because it's fall he'll usually layer up shirts with long sleeves or a hoodie.
Paige always wears either skirts or shorts, with leggings when it's cold, and cute sweaters or blazers. She wears black boots, and likes chunky scarves and socks. And she always wears her hair in a different style, because she thinks it's fun and cute.
Levi almost exclusively wears sweaters, specifically ugly Grandpa sweaters in tacky patterns and weird colors. He will layer it over a v-neck shirt or a button-up, but never wears ties or anything. He prefers skinny jeans and brown dress boots.
With those rules in place for myself, it's actually really easy to come up with clothes for them to wear. And each character has an associated color (Skyler wears red, Levi wears blue, and Paige wears pink and purple) that carries over most of their outfits for visual consistency, but it also means that I can dress Skyler in blue and Levi in red when they're thinking of each other, and that visual cue is pretty apparent.
Over the years I have subscribed to a ton of clothing magazines and when u go to design wardrobe for characters, I make a Lil sheet that has a cutout of the character and then cut and paste outfits I think would suit them. If I want to really mix it up I use this neat fashion manual I have that tells you what every type of cut of clothing I could think of is called and that's really helpful too when I want to find a reference for an outfit. I also really like to see what kind of street fashions folks are wearing around the world to get some inspo too. :3! But yeah, it can be work haha.
The strawberry dress challenge also got me trying out a new style of dress for the protagonist of my comic, so gonna throw challenges in the mix too!
