Even if the customer isn't always right, it is good business to treat customers fairly and generously.
Be available. You don't have to approach each customer that comes in the door, but do have staff on hand and ready for whatever questions or services the customer may need. If it is too much trouble to track down help, the customer will likely simply leave. If you have a website, don't make contact information difficult to find; provide your phone number and e-mail address in an easy-to-find location. And then, make sure you answer the phone and your e-mails.
Be fair. Have good return policies; if a customer returns with news that a cantelope was rotten when cut, or a knife is dull, or a coat's zipper is broken, give her a free cantelope, and do what you can to providen inexpensive knife-sharpening and zipper-reparing services. One of the challenges of maintaining a business is maintaining customers; give them a reason to keep coming back and to recommend your business to friends.
Follow through on promises. If you can't keep a guarantee, don't make it. Be straightforward and truthful with customers.
Invite feedback. Make time to listen to your customers: they are what keeps your business alive after all. Stay aware of what their needs and wants are, and tailor to those accordingly.
Always be helpful. Don't be impatient or rude to a customer; treat them like human beings above all, and not as your profit provider.
Train staff well. Train your staff to be knowledgable, helpful, and polite. Customers are seldom impressed by employees who don't know what is going on, or are standoffish and discourteous.