How do people get jobs in the movie industry?

I was speaking to a young woman who said that she was looking into a career in fashion. She said that her dream job would be to work as a costumer for a movie. I was trying to offer some sage advice when it occurred to me that I have no idea how such people get hired. I know actors go to auditions, but there are a whole lot of non-actors who show up in the credits at the end of a film. I am sure that once they have worked on one film it is easier to get hired for another, but how do they their initial break?