Anyone who says that prostitution is the world's oldest profession is forgetting that someone first had to make a sale. Ever since, salespeople have been trying to find better ways to convince customers to buy.

Hucksters try tricks or schemes that may work for a while, but caveat emptor eventually wins the day. On the other hand, there are key sales principles that have proven successful in winning and keeping customers throughout history.

You may want to make these six important customer-oriented concepts–which come from a half-dozen of the world's top sales and marketing icons–part of your company's core philosophy.

1. "Understand their needs better than any other company."

Source: Steve Jobs

Apple's co-founder also commented (infamously) that his company never used focus groups to design products. That doesn't mean they didn't listen to customers. Apple regularly queries users about needs and goals. Jobs saw his job as figuring out how those desires could be fulfilled--elegantly, efficiently, and profitably.

2. "A business absolutely devoted to service will have only one worry about profits. They will be embarrassingly large."

Source: Henry Ford

It may seem odd that the automotive pioneer most famous for telling customers they could only order cars in black would be so adamant about being responsive to customers. Ford realized he couldn't please everyone all the time. So he targeted what really mattered to his customers: affordable and reliable transportation.

3. "If I had to run a company on three measures, those measures would be customer satisfaction, employee satisfaction and cash flow."

Source: Jack Welch

 One of the champions of so-called Six Sigma metrics, Jack Welch understood that the primary goal any such program needed to be an increase customer satisfaction. The GE titan realized that increased internal efficiency and product quality would be pointless if customers would not benefit from the changes.

4. "If you build a great experience, customers tell each other."

Source: Jeff Bezos

 In the social media age, as the Amazon CEO well knows, referral advertising is more important than ever. Many customers are eager to share both good and bad experiences with the entire planet. Done right, there's nothing better--as proven by the developers of the smartphone app, Instagram, who garnered 40 million users and a billion-dollar Google buyout entirely by customer referral.

5. "People do things for their reasons, not yours."

Source: Zig Ziglar

I recently had a client tell me that he "just needed customers to understand" the difficulties he was facing. I almost spit my tea. The last thing any customer wants to hear is how difficult your job is or what struggles you are having. Give them sweetness and light, as this salesman and motivational speaker understood, and they might come back.

6. "The consumer isn't a moron; she is your wife."

Source: David Ogilvy

One the of the advertising industry's original "Mad Men," Ogilvy realized that while customers may lack specific knowledge of an industry, they do know something far more important: the reasons they will buy or not. Anyone can belittle a customer's naiveté or lack of experience. Successful salespeople will patiently and respectfully give them the information they need to make an informed decision.