Your clothes are not the only thing that employers look for. Your body language, the way you present yourself in general (clean hair, fresh scent, makeup style), the way you talk. These are all things that could make or break your interview so you have to be careful when preparing.
Along side this, you will have to research the company. Some key things you should know are:
The founder of the brand; When it was founded; What style does it have (i.e urban/designer/high street); How it varies from other brands; What inspires the brand.. etc.
You should also think about the questions you may get asked. These might include:
What are your availabilities?
Why did you chose this brand to apply for rather than any others?
How would you best describe yourself?
What do you know about the brand?
Where do you see yourself in the company in 2/5/10 months time?