Make sure your customers leave with smiles on their faces. Acknowledge every single customer. There is nothing worse than being ignored. Be sure no matter how busy you are that you at least acknowledge, by eye contact or hand motion, each and every customer that walks through the door. Let them know somehow that you won't ignore that they need help.
Customers are far more forgiving and patient when they have been acknowledged.
Never qualify customers. First impressions are everything. Not so in sales. Don't let bad vibes, age, clothing, accents, race or mannerisms predetermine a customer's worth. Judging a customer in these ways sets up a dangerous precedent that salespeople can ill afford to adhere to.