Kenneth K. Lam / Baltimore Sun / MCT / Getty Images Women wait in lines to purchase 40-inch HDTVs at Target during last year's Black Friday sales.

Stores aren’t even waiting for Black Friday this year to kick off the holiday-sale madness, but there are some things shoppers should keep in mind before they scarf their turkey and hit the sales. Retailers rely heavily on Black Friday to boost their sales figures, more so today in a lackluster economy. That means their usual tricks will be deployed, like jaw-dropping prices on items available in such limited quantities that it’s almost inevitable you’ll have to settle for a more expensive alternative. But that’s just the tip of the iceberg.

Stores are getting increasingly savvy at finding ways to get you to break out your credit card while making you feel like you’ve scored the steal of a century. But if you wouldn’t believe in one-of-a-kind, once-in-a-lifetime deals in March or May, you shouldn’t let your guard down now. Here are six things you should keep in mind during the holiday shopping season.

(MORE: Black Thursday: Will Shoppers Really Show Up at Stores on Thanksgiving Night?)

1. MSRPs are meaningless. A lot of Black Friday deals tout huge percentages off the “manufacturer’s suggested retail price,” but these can be misleading. MSRPs and non-sale prices can vary from retailer to retailer, or the retailer may even list a higher MSRP than what’s on the manufacturer’s own website. In some cases, stores will already have the item in question for sale below the stated MSRP.

An example: Kohl’s Black Friday flier advertises an 18-piece set of Cuisinart knives for $59.99, down from a “regular” price of $169.99. But a shopper on Kohl’s website now can get those knives for $119.99. The Black Friday price is still cheaper, but it’s not as good a deal. The Cuisinart website indicates that the knives aren’t available directly from the manufacturer but are sold through Kohl’s. It lists the price at $99.99. Plugging the model number into Google also turns up the same knife set at a handful of other retailers, priced in the $90 to $100 range without any special sale.

(RealSimple.com: Best Days for Holiday and Christmas Online Shopping)

2. You might have to mail in a rebate to get the deal. Many stores advertise fat Black Friday discounts, then mention in small print below the eye-catching numbers that you have to mail in a rebate to get the deal. Here’s the thing: You probably won’t. According to an academic research paper titled, “Managing Mail-in Rebate Promotions,” only about 40 percent of rebates on consumer electronics get redeemed, and the rate is even lower for other categories. The paper says, “As explained by one retailer, ‘Manufacturers love rebates because redemption rates are close to none. They get people into stores, but when it comes time to collect, few people follow through. And this is just what the manufacturer has in mind.'”

What’s more, the fine print on a lot of the Black Friday ads stipulates a maximum of one rebate per item, per address. So if you want to buy two friends or family members the same gift, you could be out of luck if you have to fill out a rebate to score the sale. (Plus, this means you’ll have to give a gift with the UPC code cut out of the box, or else ask the recipient to save the bar code for you.)

3. Applying for a store credit card could backfire. Stores use Black Friday to sign up new accountholders for their branded credit cards, often by dangling an extra 15 or 20 percent discount as incentive for doing so. But that discount isn’t free; it’s shaving points from your credit score, which could cost you big-time if you plan to take out a loan in the near future.

(MORE: Black Friday Starts on Thursday This Year at Walmart)

The first way your score is impacted is by the inquiry into your credit the merchant makes when you apply. While a shopper with a long credit history might only see their score dip by a few points, “Someone with a young credit report might see their score drop 15 to 20 points,” warns John Ulzheimer, president of consumer education at SmartCredit.com. Adding new accounts lowers the average age of your accounts, which also lowers your score. Ulzheimer says that for people with short credit histories, the addition of a new account could drop their score by 20 to 30 points. “Opening several retail cards to take advantage of several discounts will compound the damage,” he adds.

4. That doorbuster might not be the “real” thing. Some products advertised at deep discounts on Black Friday are what’s known as “derivative products” — items made by the same manufacturer that look almost like the standard model but with a few very important differences. Consumer Reports highlighted derivative electronics in a 2009 investigation and found that these products are generally manufactured more cheaply, sometimes with less-expensive components, and that they are generally missing a few features present on the standard models. The magazine says big-name companies like Sony and Samsung have made derivative TVs in previous years; we’ve seen laptop and printer deals from brands like HP in this year’s Black Friday ads that appear to be derivatives, as well.

How can you tell if you’re getting a derivative or not? The model number is often a tipoff; it will be slightly different. Or just do an online search for the model number to see if distribution is limited to a single retailer, which could indicate that it’s a product made specifically for that company in order to meet a doorbuster price.

5. You can get some of those deals now. Sometimes, you can do better — even at the same retailer — without waiting for Black Friday. This year’s Black Friday ad from Staples includes an 8GB flash drive made by SanDisk for eight bucks, but it’s currently available on the Amazon.com marketplace for as little as half that. This item also falls into the “useless regular price” category; the Black Friday ad says the regular price is $29.99, but you can buy a 16GB version of the same flash drive for $19.99 on Staples’ website, and the 8GB model is available at numerous other places for well below the $30 price point.

6. Credit card price protection can help you — sometimes. If you buy a product and then find it on sale somewhere else within a certain time frame (generally 60 days), see if you qualify for the lower price. Your first line of defense should be the store at which you bought the now-cheaper item. Bring a copy of the ad, since some stores will reimburse you the difference on the spot. If not, document the lower price and get in touch with your credit card company.

Some credit cards come with a price matching guarantee, although the degree to which this will help your Black Friday retail adventures is limited. Verbiage in price protection terms and conditions spells out pretty specifically that the protection won’t apply for sales in which the retailer advertises a limited supply. The rules are a little blurrier when it comes to Black Friday deals that last for a very short window of time but don’t warn about limited supply. “It all depends on how a retailer would word this,” says a spokeswoman from Capital One. In other words, it’s worth looking into.