Image: Couple shopping for car © Don Mason, Blend Images, Corbis

Buying a new car is one of the most stressful financial situations you're likely to encounter. There are many reasons for this. After all, this is one of the biggest purchases (aside from a house) that you'll make, and you want to get a good deal.

One big obstacle for many is dealing with salespeople who try any number of tricks to get you to spend more than you planned.

Understand the process

While there are honest car dealers out there, dealing with a crooked dealership can leave a very bad taste in your mouth. If you know the tricks of the trade, though, you can go in prepared to get a fair deal for your next car.

Here are some of the most common tricks you'll encounter when buying a car or trading one in:

Trade-in tricks

Trading in your old car can help offset a portion of the purchase price on your new car, but it's rarely the best deal. In most cases, you'd be better off selling your old vehicle yourself and then putting the proceeds toward the new car. It's more legwork, but if you're patient, you can get a much better deal.

Inflated trade-in values. If you do decide to trade in your car, the salesperson might come back with a fantastic offer -- perhaps much more than any other dealer would offer. Seeing as this price is much better than what you can get elsewhere, you may go ahead and start making a deal for your next car.

The problem with this situation is that, all too often, the sales guy will eventually claim that the trade-in price was rejected by his manager. He'd like to keep the deal with the new car but will offer a smaller amount for the trade-in. At that point you may be so emotionally invested in the process that you're likely to accept his offer.

Don't fall for this trick. Stand up, start moving toward the door, and ask the salesperson to lower the new car's price correspondingly. If they won't budge, just walk away.

Holding your keys. Another possible gotcha occurs after you hand the dealer the key for your trade-in. There have been stories of dealers keeping potential customers in the showroom by "misplacing" their keys. Our friends had it happen to them. Upset with that old trick, they wound up leaving without purchasing anything.

Scams when buying a car

The biggest advantage you can have when buying a car is knowledge. Don't go into a dealership without knowing what kind of car you're looking for and how much it should cost. If you're prepared, you'll be able to spot outrageous offers and comfortably walk away.

Foursquare -- bad math. If a salesperson asks how much you're willing to pay per month for your car, watch out! Some dealerships coach their salespeople to pull out a foursquare chart to maximize their profit by focusing only on your monthly payments.

Payments can be reduced by extending the length of your loan, or by including a balloon payment. Know what you're looking for and tell them what -- and how -- you want to buy. Use Kelley Blue Book values or price data to see what your dream car is worth and how much people are paying for in your area.

Instead of getting into the tricky math at the dealership, focus on the bottom line -- the total price you're willing to pay for the car. If you need financing, you should consider lining it up on your own rather than including that in your negotiations.

Lowball first offer. Similar to the overly generous trade-in offer, a too-good-to-be-true price on the car you want to buy isn't that uncommon. As you rush to close the deal, however, the manager inevitably steps in and says that the price is too low. The dealership can't sell it to you at that price because it would "lose money." Don't fall for it.

If a salesperson has been at the dealership for any reasonable amount of time, he'll know what sort of deal will pass muster and what will get rejected. Simply tell him you understand, but that you'll need to find a deal that is better suited to your situation. Then get up and head for the door. If he capitulates, great. If not, keep walking.

Push you to pay more. Finally, be on the lookout for attempts to add extras to your purchase. Maybe salespeople will claim that the bank requires you buy a warranty before they'll approve the loan. Another tactic is to use your credit score as an excuse to get you into a ridiculously high-rate car loan. They may be right that your score won't get you the best interest rate, but it doesn't mean you have to take the worst.

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Prepare for this tactic by knowing your credit score. You should also check with your local bank or credit union to see what sort of interest rate you can qualify for. Knowing these things in advance can help you evaluate any deal you come across.

Note from Nickel: My preference is to avoid the showroom entirely, and to negotiate the deal via email and/or fax. That way you can play dealerships against each other and find the bottom of the market without racing all over town. It also keeps you out of the sales office, so you avoid any shenanigans.