Even with the holiday shopping season now starting in early November, Black Friday remains a sort of national sales holiday.

It's probably not exactly correct to call it by that name anymore. What once began strictly the day after Thanksgiving has extended itself into the early evening of the holiday itself at major retailers, including Wal-Mart and Target. From across-the-board savings to "doorbuster" deals at near-impossibly good prices, there is a level of "you can't miss this" hype that makes Black Friday shopping (even if it starts on Thursday) a major event.

This can make it easy to get swept up in the mania, buying things you don't need, don't really want, or can't afford. In anticipation of the holiday shopping frenzy, here are four mistakes to avoid on Black Friday.

Don't buy solely based on price
For the past few years, Target has been offering small appliances, including basic toasters and coffee makers for very low prices. For each of the past two years, I have visited one of the chain's locations on Black Friday, seen these deals, and purchased a few items.

"Who could pass up a toaster for $5?" was my thinking. Of course, this ignores the fact that I already own a much nicer one and that I cannot eat toast given my gluten allergy. A good deal is only a good deal if you actually use what you buy. You're not saving money just because an item has been marked down.

Don't fall for the extended warranty
If a chain store sells a customer an expensive piece of electronics at a cheap price, one of the ways it can boost profits from the deal is to package the product with an extended warranty (sometimes called a service plan). Buying this is almost never a good idea, and Consumer Reports recommends against it for a number of reasons.

Some of the reasons are obvious -- you're credit card may already offer you extended coverage and in some cases, the original manufacturer's warranty already covers you. The big reason, however, is why these deals are so good for retailers:

"Products seldom break within the service plan window," wrote the non-profit consumer review website. "Our data show that products usually don't break during the two-to-three-year period after the manufacturer's warranty expires and the service plan is in effect."

It seems tempting, and in some cases, the salesperson will push the peace of mind offered by service plans, making it seem like you would be silly to pass it up. Be strong. Say no and realize that you're not taking a risk, you're actually saving money.

Know the full price
The first time I bought a flat-screen TV, I was lured in by a deal during the holiday shopping season. I brought the 32-inch model home, took it out of the box, and promptly realized that while I could plug the traditional cable wire into it, I actually needed an HMDI cable for the best possible picture. I also learned that simply placing the TV on my then wall unit with its included stand left it vulnerable to being knocked over by both my somewhat reckless child and my two curious cats.

I got a deal on the TV, but I paid full price for the cable. I also paid nearly as much as I did for the television to buy a mounting bracket and to have a contractor mount it for me.

The TV was a deal, but the cost to actually use it in the way I wanted was much higher. When buying anything on Black Friday, consider costs like batteries, cables, carrying cases, installation, and anything else that can quickly make the "bargain" you're getting unaffordable.

Remember to comparison shop
When you've left the Thanksgiving table for the mall, it can be hard to avoid the urge to make a purchase simply because of all the effort involved in getting to the store and navigating through the huge crowds. Coming home empty handed feels like a waste of time, and that leads people to forget to comparison shop.

Amazon Price Check

Amazon's price check app Source: Author

The best move may be not making one at all, or leaving one store to head for another. It's also possible to see an item in a physical store but buy it from an online retailer.

"One mistake is assuming that something in a store, simply because it's on sale on Black Friday, is a good deal," Jim Wang, founder of the blog Bargaineering.com, told Time.com in 2014. "It might be a decent deal, but you could do better going online."

Amazon even has a price check app that makes it as easy as scanning a bar code to check if the online retailer is offering a better deal. In some cases, however, knowing where the best deal is requires comparison shopping beforehand. That's especially true when taking advantage of a doorbuster or a limited-time offering.

However you shop, it's best to be informed. Do your research before you leave the house and before you hand over your payment in a store. That might leave you empty handed on Friday, but you will have more cash in your pocket as items are delivered to your house just a couple of days later.

Daniel Kline has no position in any stocks mentioned. He is spending Black Friday at home this year. The Motley Fool owns shares of and recommends Amazon.com. Try any of our Foolish newsletter services free for 30 days. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.