WITH GAS PRICES

hovering around $2.89 a gallon for regular unleaded, who isn't looking for ways to alleviate that pain at the pump? Using a credit card that offers cash back on your gasoline purchases is one way to potentially cut costs.

Here's how they work: These cards offer cash back on all your purchases, with better rebates when you buy gasoline. But like any credit-card offer, you need to check under the hood before you sign up. "Gas, like other rewards, can just be a gimmick to get you to sign up for the card," says Scott Bilker, founder of Debtsmart.com, a web site dedicated to helping consumers manage credit cards wisely.

Needless to say, these cards will save you money if you pay off your balance in full each month. If you don't, you'll most likely be worse off using them than if you were to use cash.

Here's how to get the most mileage out of a gas card:

Look beyond introductory offers
Credit-card companies know you want to spend less on gasoline, so they've advertised some very attractive initial offers, says Bill Hardekopf, CEO of LowCards.com, a consumer information web site focusing on credit cards. Before you race to sign up, investigate how the rewards change down the road. Both the Marathon Platinum MasterCard and the WaWa Visa card, for example, initially offer 10% cash back on gasoline purchases at their branded stations. But the Marathon rate drops to 5% after 60 days, while the WaWa rate drops to 4%.

Skip station-specific cards
You need the flexibility to buy gas at whichever of your local stations has the lowest price on a given day, says Bilker. Unless you only have Exxon stations near you, or that Hess station around the corner always has the cheapest gas, you just won't rack up enough rewards to make it worth your time.

To find the best nonbrand specific cards, we scoured the web. There aren't many out there, but here are two we liked:

Consider a generic cash-back card
Getting a solid rebate on your gasoline purchases is great, says Bilker, but it is likely those purchases are just a sliver of your total credit-card bill. So crunch the numbers: A cash-back card may be a better deal. These cards offer a slightly higher rebate rate for all your purchases 1% to 5%, depending on the retailer, compared with up to 1% as "other" purchases on gas-reward cards. Discover Platinum, for example, offers 1% back on purchases, plus 5% rebates in revolving award categories that include gasoline and travel. Plus, you've got the flexibility to spend those savings on whatever you like.