By Dean Rieck, direct mail copywriter
Apart from food, water, shelter, basic clothing, and warmth, most purchases today are discretionary. People buy stuff they want, not necessarily stuff they need. So selling is largely a matter of understanding the psychology of your customers. 
You don’t have to be a PhD to figure people out. But you do need to know a few key psychological concepts related to direct marketing and buying.

8.) People love to buy. There is a common myth that people don’t like to be “sold.” That’s not true. People love to be sold. They love to discover wonderful new products and experiences. In America, and increasingly around the world, people see commerce as a natural and appealing activity. What people don’t love is to be cheated or tricked. Therefore, it can be helpful to change your analogy of the marketing process. Instead of “selling” to people, try to “help” them. Sell good products, make appealing offers, and treat people fairly. That’s a surefire formula for success.

9.) People are naturally suspicious. It’s true that there’s a sucker born every minute, but most people are moderately skeptical of any offer. You can never predict the level of suspicion any particular person has, so it’s usually best to back up all claims with evidence that is meaningful to your prospect, such as testimonials, survey results, authoritative endorsements, test results, scientific data, etc.
10.) People don’t read in a predictable, orderly way. Advertising is not literature. People seldom read in a linear fashion, beginning to end. They skip around, try to find meaning quickly, and usually don’t read closely unless they are interested — and even then they may jump around. You have to write and design for this random reading, assuming that it may begin or end at any number of points. 
11.) People are looking for something (and it’s not a product). Love. Wealth. Glory. Comfort. Safety. People are naturally dissatisfied and spend their lives searching for intangibles. At its simplest, creating selling messages is simply a matter of showing people how a particular product, service, or cause fulfills one or more of their needs. (I have a theory that what many people are really looking for is “control.” People are happiest when they can easily get what they want and feel protected against losing those things — in other words, when they have control in their life.)

12.) People shop by mail because of convenience and exclusivity. One survey revealed the reasons people shop by mail. In order, they are: Convenience, Exclusivity of the Product, Variety, Fun, Price, Quality, Service. Convenience and exclusivity were the leading reasons by a large margin. The humbling lesson? If your customers could easily find the things you offer at a nearby store, that’s probably where many would buy them. So if they are not buying from you through the mail for sheer convenience, they’re doing it because they can’t find the item elsewhere (or just don’t know where to look). It’s wise to emphasize the convenience and exclusivity of your products and services.
13.) Many people like to see it and feel it before they buy it. Some people never shop by mail (or by phone, TV, or Internet) because they can’t examine the merchandise before making a commitment. Some items, such as books and magazines, are tangible and familiar enough to sell easily through the mail. There is little doubt about the physical quality. Other items, such as clothing or food, may be a harder sell — at least until people have a satisfactory buying experience — because quality may be variable. You can’t easily judge the stitching of a blouse or the smell of a steak from a photograph. Think about how people buy things in stores and ask yourself if there is some element of that experience that is missing from your sales message.
14.) People are bound by “reciprocation.” There is an overwhelming urge to repay debts, to do something in return when something is done for us. This deep-seated urge is so strong, noted paleontologist Richard Leaky has said that it is the very essence of what it means to be human. And sociologist Alvin Gouldner points out that no society on Earth escapes the reciprocity principle. A classic example of this principle used in direct mail is how the Disabled American Veterans increased response to their appeals by giving away personalized address labels. The standard appeal was pulling an 18% response. When the address labels were given away, the response rose to 35%. The lesson? Be generous. Give people something.

15.) Most people follow the crowd. Most of us are imitators in most of what we do. We look to others for guidance, especially when we are uncertain about something. We ask, “What do others think about this? What do others feel? What do others do?” Then we act accordingly. This is why testimonials and case histories are so influential.
—Source: Dean Rieck is a leading direct mail copywriter. For more copywriting and selling tips, sign up for Dean’s FREE direct response newsletter and get a free report, 99 Easy Ways to Boost Your Direct Mail Response.