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'Shark Tank' investor Kevin O'Leary: These are the 2 worst mistakes people make on Black Friday

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If you plan on joining the Black Friday crowds this weekend, you may want to prepare by getting enough sleep, staying hydrated, learning where the best sales are — and by listening to the pros about how to shop smart.

Best-selling author, finance commentator and ABC's "Shark Tank" investor Kevin O'Leary tells CNBC Make It that shopping on Black Friday can be great, as long as you don't make these two mistakes.

Don't buy things you don't need

"I never recommend buying something just because it's a 'good deal,'" O'Leary says. "If you're spending money on something you wouldn't have bought otherwise just because it has a '50 [percent] off' sticker on it, you're still throwing away your hard-earned dollars."

So don't buy something just because it seems to be on sale. The impulse may seem good in the moment but regrettable later.

Instead, make a list and a reasonable budget, and stick to them both.

"If you have a shopping list of items that you know you'll be purchasing over the next few months regardless of price," O'Leary says, "waiting for Black Friday sales is the most financially responsible option."

Don't use credit cards

And, much like self-made millionaire and fellow "Shark Tank" investor Mark Cuban, O'Leary suggests you choose to pay with paper, not plastic, at the check-out aisle.

"My advice, as always, is: NEVER SPEND ON CREDIT," says O'Leary.

Americans hold more than $1 trillion in credit card debt, and one survey finds the average card user is more than $5,000 in debt.

Plus, using credit can come with additional costs such as high interest rates, surcharges and other fees, like the extra dollar or so a movie theater charges you to buy tickets in advance with a card.

Instead, consider using cash. Research shows it feels more painful to watch bills disappear than it is to swipe away digital funds, so using cash could lead to spending less.

If you plan to use plastic, though, don't spend more than you can afford. If you can't pay your balance in full each month, financial expert Tom Corley says you need to cut back, perhaps, in this case, by choosing less expensive gifts. Check out these 25 gifts for under $25, for example.

Black Friday shopping can be an exciting experience and a financially savvy choice — and, if you stick to these rules, you can make it through without breaking the bank.

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