WASHINGTON — Michael Cheng and his wife, Nicole, each have four credit cards in their wallets. A Post-it note on the top left corner of each card lets them know which one to use for what.

The Chase rewards card is for groceries, gasoline and drugstore purchases. The Chase business card is for gas stations, home improvement and office-supply stores and restaurants. The Discover card is for restaurants and movies. The Farm Bureau Bank card is for everything else.

In return for their purchases, they get checks or gift cards: 5 percent cash back from the Chase rewards card, 3 percent from Chase business, 5 percent from Discover and an amount equal to 1 percent from the Farm Bureau.

The Chengs, who have a stack of other cards, keep track of the cash rewards on a spreadsheet: They made $1,121 in 2006 and $1,093 so far this year. They don't pay interest on their purchases because they immediately pay off their balances. They pay no annual fees.

"We get the sense that we figured out how to play the game and we're playing it our way," said Nicole Cheng, 43, of Centreville, Va.

That game is played by so many that there are numerous Web sites such as www.CardWeb.com, www.Credit-reviews.com and www.Creditcardgoodies.com that rate the programs and provide venues for cardholders to seek and give counsel. Some of the most avid users are those who get cash back, a segment of the industry that has grown rapidly in recent years.

If you're going to play the rewards game, beware: The credit card issuers are certainly up to the challenge, several consumer advocates said.

"There's a small number of people who can game the system, but for every consumer who games the system, there's probably 25 consumers who are ratcheting up credit card debt and never are redeeming the rewards," said Ed Mierzwinski, consumer program director for the U.S. Public Interest Research Group.

Be prepared to spend a lot to make a lot — or a little. So far this year, the Chengs have spent $47,800 to get $1,093 in rebates. "Mind-boggling," Michael Cheng, 42, admits.

The only way to make the rewards cards rewarding is to be disciplined. Card issuers divide their clients into two categories: Transactors pay off their balance each month; revolvers carry balances.

If you're a revolver, don't bother with a rewards card. Whatever you earn is not going to make up for what you pay in interest. The cards with the best rewards tend to have higher rates.

"If you carry a balance on your credit card, a 1 percent reward does not in any way offset a 15 to 25 percent APR," Mierzwinski said. "For consumers who carry balances, they rationalize that at least the reward is cutting it a bit, but you're much better off getting the best rate you can."

Be sure to understand the rules, consumer advocates and credit card experts said. Many people don't. In a survey conducted last year by Braun Research of Princeton, N.J., one-third of 1,000 cardholders surveyed said they either don't read or don't understand the fine print contained in credit card offers they receive by mail.

"The big print giveth and the small print taketh away," said John Ulzheimer, president of consumer education for Credit.com.

Many card issuers reserve the right to void rewards if a payment is late, and they can limit the amount of cash or number of points redeemable. Or they might require that the rewards be redeemed within a certain time frame. Sometimes, rewards can be hard to claim, particularly airline miles.

Working hard to entice customers, card issuers have increased the number of rewards programs. They have also gotten more creative with the programs.

Take the Citi Home Rebate Platinum Select MasterCard. Cardholders can use the points to pay down their mortgages. For big spenders, the recently launched Sotheby's MasterCard offers a helicopter tour of California's wine country or the use of a Sotheby's auctioneer to conduct a charity auction of your choice. If you're a college student, you can earn points for having a good GPA and redeem them for plane tickets, CDs and other gifts. And with gas prices skyrocketing, credit cards that offer cash back at the pump are popping up almost everywhere.

How many does Warren Wilkins have? "I don't even know," said Wilkins, 69, a retired lawyer who lives near Richmond, Va. He believes he has six to 10, but actively uses three. "It's great to pick up $1,000. It's just a freebie," he said.

Credit card experts and consumer advocates said some card companies are not as generous as they used to be or are making the programs more complicated. Gone are the days of 5 percent cash-back deals. Now, 1 percent cards are more the norm. Other companies have created tiered programs as a way to encourage spending, giving a higher percentage of cash back the more the consumer charges.

Citigroup, for example, cut rewards earned by holders of the Citi Dividend MasterCard to 2 percent from 5 percent. American Express slashed rewards points by half for everyday purchases at gas stations, drugstores, supermarkets and other places.

"It seems that a lot of the issuers maybe were taken aback that so many cardholders were maximizing the rewards and the banks were losing money on them," said Joseph Ridout, a spokesman for the advocacy group Consumer Action.