4. Beware of the yo-yo finance scam.
You sign all the paperwork, get the keys to your shiny new car and drive it home, assuming the deal is done. A few days or weeks later, someone from the dealership calls and says they were unable to get the financing approved at the agreed-upon price.
You must return the car to the dealership, they say, or negotiate a new loan at a higher interest rate. If you don't, you could lose your deposit and trade-in, and you may even be charged a rental fee for the time you had the vehicle. Faced with this situation, most people cave.
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How can they do this?
"Most dealers don't consider the sale final until the money is in their account, and that could be anywhere from a few hours to a couple of days," said Chris Kulka, senior vice president at the Center For Responsible Lending.
Chances are this was disclosed somewhere in all the paperwork you signed in the dealer's financing office.
"The only way to protect yourself is to either get your financing elsewhere or tell the dealer that you're not going to take the car until the financing is deemed final," Kulka said.
5. Don't get hung-up on the monthly payment.
A lot of people assume that if they can afford the monthly payment, they got a good deal on the car.
"That's a huge mistake," said Jack Gillis, author of "The Car Book 2014."
Buying a new car typically involves three negotiations. There's the price of the vehicle, the value of your trade-in and the financing. And they need to be kept separate.
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"If you just look at the monthly payment, you'll have no idea what you're being charged for the car, you won't really know what you're getting for your old vehicle and you won't know what the interest rate really is," Gillis warned. "The artificially low monthly payment will disguise the fact that you're paying more than you should for the car and financing and getting less than you could for your trade-in."
The salesperson will probably ask how much you can afford to pay each month—they're trained to do that. Gillis says there's no need to answer.
Keep in mind: If you are pre-approved for the loan before you head to the dealership, you can concentrate on haggling for the lowest price for the car and highest amount for your trade-in without the added pressure of negotiating the interest rate and other details of your loan.
—By CNBC contributor Herb Weisbaum. Follow him on Facebook and Twitter @TheConsumerman or visit The ConsumerMan website.