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Have you ever bought a product that turned out to be useless or voted for a candidate you ended up not liking? Even the smartest people fall prey to sophisticated persuaders ... but you can learn to resist their influence. The key is to understand how experts in persuasion exploit six powerful instincts.

When someone does you a favor, even one that is of little value or importance ... you feel obligated to reciprocate.

Example: In one study, when restaurant waiters presented the bill along with two complimentary mnts for each diner, tips increased by 14%.

Retailers that offer free samples are taking advantage of the same instinct to reciprocate the consumer feel indebted. Even people you know may use the same strategy. A neighbor may offer to help fix your car, then ask to borrow in a week later.

Note that in the above, I am not saying that there are many good, decent people who are helpful for it's own sake (for the sake of just being helpful). But one has to be alert to that "other kind" of helpfulness.

Just as it's human nature to reciprocate, it's instinctive to cling to a position that we've taken.

Example: A charity for the disabled nearly doubled contributions in one neighborhood. First, it asked residents to sign a petition supporting the disabled. Then two weeks later, the charity solicited contributions. Most residents dug more deeply into their pockets because they wanted to be consistent with how they had responded to the petition.

Again, I should never denigrate charities but when giving, give graciously; not from sneaky marketeering.

Almost everyone makes decisions by observing choices that others have made.

Example: Psychologists from City University of New York found that when one person on a sidewalk stared at the sky, only 4% of the passersby also looked up. When the experiment was repeated with a large group staring at the sky, nearly all passersby stopped to look up.

TV infomercials routinely capitalize on this instinct by announcing, "If operators are busy, please call again" ... a subtle way of telling viewers that the product is so good that other buyers are jamming the switchboards.

Though we don't always realize it, we're more likely to be influence by people we like or identify with.

Example: Tupperware has achieved worldwide success with its home-party program, in which customers buy from friends rather than from unknown salespeople.

Most of us instinctively trust people who display symbols of expertise or authority ... an auto repairman in a mechanic's uniform or a doctor with a wall full of diplomas, for example.

Example: University of Texas researchers found that pedestrians were more likely to cross a street against the light if they were following a man who was wearing a suit and tie. On the other hand, as a New Yorker, how many people in Texas were suit and ties?

We're influenced by the prospect of scarcity and deadlines. Businesses take advantage of this with such phrases as "one of a kind," "limited supply" or ":limited time only."

Example: Oldsmobile sales boomed when General Motors announced that production would soon halt.

When you are under pressure to make a purchase or respond in a certain way, delay your decision. Use the time to decide whether you need the product or should get more information. Are you tempted to buy something because you like the salesperson or because supply is "limited"? Have you checked a repairman's or contractor's references? Getting more information is the key to resisting sophisticated persuasion tactics.

Most of the above is from an interview with Robert B. Cialdini, PhD, Regents' Professor of Psychology at Arizona State University, and author of Influence: Science and Practice (Allyn & Bacon).