I have never worked in a clothing store, but in undergrad school some of my classmates did a riveting presentation about what it's like to work at...Hollister? It was one of those preppy clothing stores that I personally would never go in, so I knew nothing about it before this presentation. It was fascinating and I remember it well years later.
They said that every clothes store has a culture and you should blend in with that culture, mostly by the way you look. If you don't blend in, you can get a job stocking shelves or working in the back, which might be a better job anyway.
The "look" of the preppy store is to have shoulder length or longer hair, to be in shape or slender, and to wear natural colored makeup. If you're a guy, it's to be in shape or slender and to have trendy, but not outrageous hair. No piercings or visible tattoos. A light tan is preferable if you're white. Your clothes should be from the store you're interviewing for. This last is really important.
Obviously this would be different for different stores. If it's a plus sized store they would prefer to hire plus sized people. If it's Journeys or one of those snappy trend stores, funny colored hair and piercings and stuff are preferred. You get the idea.
I never thought clothing stores would be interesting until this presentation. If you find that you'd like to learn more, there's an author named Paco Underhill who writes about retail science. Also quite interesting.
Good luck! Let us know how it goes!