This holiday season, the hottest trend among retailers isn't found on a store shelf. It's taking place at the cash register.

Major retailers, from Best Buy to Toys R Us, are promising to match their competitors' prices. Generally customers just need to bring in an advertisement or printout to prove that the same item is available elsewhere at a lower price. In some cases, shoppers can come back with a receipt and get a refund for the difference if the price of an item they bought fell.

Best Buy Co. Inc., Target Inc., Wal-Mart Stores Inc. and Sears Inc. offer price matching to customers all year round. But what's different now is that Best Buy and Target are matching online retailers such as Amazon.com for the first time. That's a big deal, since online prices tend to be lower than those at stores.

Shoppers will be able to save some extra money, but they're going to have to read a lot of fine print to do so.

"Price matching sounds good, but there are so many exclusions, it sometimes isn't as good as it sounds," says Edgar Dworsky, the founder of ConsumerWorld.org, which tracks deals for shoppers.

For instance, Target limits the number of online retailers that it will price match against to just five. Best Buy has selected 20, but only matched online prices from Nov. 1 to Nov. 17 and will start again from Nov. 27 to Dec. 24.

Toys R Us is offering price matching for the first time and will only match prices that customers find in other brick-and-mortar stores. Walmart also matches against in-store prices.

Toys R Us, Best Buy, Sears and Target say they will match prices found on their own websites. It's not uncommon for retailers to offer steeper discounts online than in their actual stores. (But Toys R Us says it won't match prices on its own website if the item is marked as an "online-only price.")

Even the most experienced bargain hunters can get tripped up by all the rules. But shoppers can save some money if they're diligent.

"It really is a way to save money and shop at the store you want instead of one that's inconvenient," says Dworsky.

Here are five ways to get the most out of price matching offers:

1. KNOW THE POLICY

If you want to take advantage of a price match offer, read the store's policy closely. You can find the guidelines on the store's website.

Print out the policy and bring it with you. Having a hard copy will be helpful if you need to argue your case.

"Know their policy backwards or you may be bluffed into thinking something doesn't qualify," says Dworsky.

2. BRING PROOF

Always bring the advertisement or the printed web page for the item you want to price match. Walmart doesn't require bringing the ad because it says cashiers have access to all local advertisements. But Dworsky recommends bringing ads in anyway. If there's any confusion, you'll be better prepared to make your case no matter where you shop.

The cashiers and customer representatives are always looking for a reason not to approve the transaction, says Dworsky.

3. SAVE RECEIPTS

Some retailers will give you money back if you see a lower price after you buy an item. Keep a hold of your receipts and, particularly for big-ticket items, continue to look for lower prices.

Best Buy will issue refunds until the end of January. Toys R Us lets you seek a refund up to seven days after buying an item. Sears customers can get a refund after 14 days. Target is letting customer's price match against brick-and-mortar retailers until Dec. 24 for any item bought after Nov. 1. You can only ask Target to match the price of an online retailer until Dec. 16.

4. GO STRAIGHT TO THE CUSTOMER SERVICE DESK

Many retailers have hired cashiers specifically for the holiday rush, so the new employees may not be up-to-speed on the store's price matching policy. Heather Wheeler, who runs savings website TheKrazyCouponLady.com, recommends handling the transaction at the customer service desk instead on the cashier.

"(Those staffers are) trained a little more and are more knowledgeable," says Wheeler.

5. LOOK BEYOND RETAILERS

You can also price match depending on how you pay. EBay Inc.'s payment processer, PayPal, promises to match a lower price if you've already made a purchase using the service. That includes airline tickets. PayPal will match the prices of retailers that don't let customers use PayPal, however. Just fill out a form and upload a receipt when you find a lower price. PayPal will give you back up to $1,000 for all purchases made until Dec. 31.

You should also ask your credit card company to see if it offers price matching. It's rare, but there are a few cards that do.

Citi just launched a program for its credit card holders. Called the Citi Price Rewind program, it promises to do the work for you. Register your purchases made on the Citi credit card online and it will send you a check for the difference if it finds a lower price from an online retailer.

The program is aimed at pricier purchases: It will only issue a discount if the price difference is $25 or more. Citi will give you the amount up to $250 for each item, and up to $1,000 a year. Of course, you're going to need pay your credit card bill in full and not incur interest charges to truly make this a deal.

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Joseph Pisani can be reached at http://twitter.com/josephpisani