Cap on rewards
Nine of the cards in our research have a cap on the cash back you can earn. Some of these cards have a monthly cap, some have an annual cap and others have both. In our survey, caps generally range from $300 to $600 per year.

Obviously, make sure the cap won't hamper the rebates you can earn. Crunch the numbers to see how much you would have to spend before hitting the cap. The cap may not impact you at all unless you're a big spender.

Tip: "All other things being equal, go with one that has no caps," says Curtis Arnold, founder of CardRatings.com. Beyond that, go for one with a high cap -- at least $500 annually.

Arnold suggests that one way to deal with a lower cap is for you and your spouse to each get the same rebate card. Having two cards doubles your cap.

Expiration date on rewards
Six of the cards we surveyed had expiration dates, typically three to five years from the date the reward was earned. Clear from American Express, whose cash back comes in the form of an American Express gift card, expires one year from the date of issuance. Discover's Open Road and More cards specify that if the account is closed for any reason or goes inactive for 18 consecutive months, or if you don't make the minimum payment on time for two consecutive periods, you lose your cash-back bonus.

Curtis Arnold, founder of CardRatings.com, says it's a good idea to find out what could trigger the forfeiture of earned rewards. Usually becoming delinquent on your account results in loss of rewards.

Tip: Arnold says many rebate cards don't have expiration dates, but it can be an issue with points-based cards. For the ones that do impose a shelf life on earned rewards, make sure you would have enough time to earn and redeem cash back.

Excluded merchants and purchasesTen of the cards we surveyed have restrictions on which purchases count toward a rebate. Some restrictions are pretty straightforward -- anything that's not a purchase, such as a cash advance or balance transfer, doesn't count toward a rebate, but other exclusions are not so obvious.

For instance, purchases made at Wal-Mart using one of the Chase Freedom cards won't be eligible for the 2 percent cash-back rebate. Purchases made with a Citi Dividend card at a department store, such as Macy's, won't earn the 2 percent rebate. Department store purchases don't count toward that enhanced cash-back rate.

Tip: Always hunt for the terms and conditions of the rewards program and scour it for merchant restrictions. You might be surprised to learn what doesn't count toward rebates. Find as much information as you can on the card issuer's Web site and sites such as Bankrate.com. If you still can't find the information, Curtis Arnold, founder of CardRatings.com, suggests calling customer service. Ask for a manager if the representative can't answer your questions or tells you the information will be disclosed in the agreement when you receive the card. "I hate that answer," he says. "That's not an acceptable answer, but you'll hear that a lot."

If you still can't get the answers you want, consider taking your business elsewhere.