Here are some favorite lines from an article by Ellen Rohr called Financial Dos And Don’ts Lead To Better Business:
Do understand that you are in business for the money. 
Don’t be ashamed about that.
Do whatever it takes to make a customer happy, even if it means giving them all their money back. If you honestly think you are being taken advantage of, then …
Don’t work for that customer again.
Do spend more time asking questions than pitching your product.
Don’t make a sale if it isn’t in the best interests of the customer.
Do feel free to say no to customers who won’t let you make a profit. Don’t use low prices as your selling advantage. The advantage disappears the moment someone dumber than you offers a lower price.
Do apologize if you are wrong.
Don’t hold a grudge.
