Many credit unions require members to use their checking account debit card a minimum number of times per month, says Martin Kelly, senior vice president of marketing and business development for US Federal Credit Union in Burnsville, Minn.

The reason for the requirement is that for credit unions to earn revenue from checking accounts (and pay out high yields), there needs to be a good amount of account activity, which can be accomplished through debit card use, he says.

For people who normally pay with cash or a credit card, it can take some getting used to, says Paul Hendley, a member of First Community Credit Union in St. Louis. He says he joined a local credit union to take advantage of its high-interest checking offering but had to start using a debit card at least 12 times a month to qualify.

"At first, I worried that I'd have to run out and buy things just to satisfy the debit card requirement, which would defeat the point of getting a higher interest rate. But I found out that doing 12 transactions for the month with normal shopping was easy," Hendley says.