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.