"Research shows that the loyal customer is spending several times more than the customer who is not loyal," says Mr. Anand of loyalty program participants. Sending loyal customers coupons or advance notices of sales is a successful sales strategy, he adds.
In addition, Aberdeen's research shows customers are four times more likely to sign up for a merchant's loyalty program if told about it at the register. Plus, customers who feel "loyal" to a store are more likely to recommend it to friends and family, he says.
If all the checkout queries seem too intrusive, politely dismiss them, advises Ms. Greenberg. "You can say, 'I know you have to ask as part of your job, but I'm not interested.' " Here's how to safeguard your privacy and pocketbook:
1. Think before joining loyalty programs. On the one hand, "rewards" are increasing, says Mr. Holman. Retailers are sweetening the pot as new technology empowers consumers to seek better deals online, sometimes right on the sales floor, using mobile devices. But some charge annual fees, which could offset savings for infrequent shoppers, he warns.
2. Don't give out your phone number. Asking for a phone number helps a merchant quickly verify whether the customer is a member of its rewards program. But consumers could be unwittingly agreeing to unwanted solicitations. Mitch Katz of the Federal Trade Commission, the government agency that manages the national Do Not Call Registry, notes an exception that allows businesses a window of 180 days to phone customers after they have made a purchase.
Mr. Katz suspects some merchants might be resetting that 180-day period each time a customer visits the store and provides a phone number. While some customers might appreciate the follow-up after buying an item or service, those who object can ask to be added to that company's own do-not-call list.
3. Don't feel obligated to give your ZIP Code. ZIP Codes simply aid marketing, Katz says. "I never give mine."
4. Beware of sales pitches for credit cards. Penalties and fees can easily exceed one-time discounts. "Credit cards are full of tricks and traps," says Greenberg of the National Consumers League. "It's not a game I would play."
Stores should suggest applying for a credit card when it's clearly to the customer's benefit, such as saving 10 percent on an $800 purchase, says Mr. Dion, the retail consultant. "But if it's a $30 purchase, I'm going to say, 'I should go through this hassle for $3? Get out of here.' "
There are examples where retailers themselves have pulled back from gathering information on their customers. After hearing about an arson case in which an innocent defendant's loyalty-card history was subpoenaed to prove he bought lighter fluid, Michelle Harrington, co-owner of Albrecht's in Delafield, discontinued the store's loyalty program.
Customers shouldn't trade privacy for weekly specials, she says. "I don't think people think about that."