You should dress smartly
Arrive confident and ready to work. They want to see you've got personality and charisma.
You should prepare by first of all reading up on the company ad before an interview go into the store and check out the items they have so when they ask you what you know about the company and the products that they specialze in you will be able to give confident and accurate answers.
Simple mathematics, e.g. knowing how to calculate a percentage. You must be bubbly and approachable, and quite outgoing
Research the company!
Even if the question isn't asked always be prepared for it. remain calm, don't go into the interview all anxious and nervous. be yourself and confident and don't be afraid to list what you can bring to the company.