Establishing purpose is an easy step that is sometimes overlooked.  People have many reasons to visit a retail store, and sometimes Sales Associates make assumptions that impede their ability to assist customers.

Why are they here?
Customers shop in stores for various reasons; it is important for you to establish that reason quickly.  This will prevent you from starting the wrong discussion or mistakenly assuming the reason for the customer's visit is based on the merchandise they're looking at.

Ask the customer a direct, clear question that establishes their purpose; this is the easiest way to make sure that you will begin the proper discussion with that person.