(1) Purchasing the wrong auto. As mentioned earlier it's important to meet your needs and not your wants. Pick an auto that fits with your budget and daily lifestyle. If you drive a great distance everyday then you'll want to choose a car that is more comfortable for long distance travel or is more economical in terms of gas mileage. If you have kids it may not be a good idea to pick a sports car.
(2) Showing your emotions in the showroom with the dealer or manager. Emotions cloud our judgment and the dealers know it. Auto dealers take full advantage of this when they see a potential client dreamy eyed and salivating over their fantasy car. Once they know this they will be much less willing to negotiate on price or terms. You'll also be more likely to accept a bad deal if your emotional. In most states they have a three day "cooling off" period where you can take the car back if you want too. (For this exact reason!)
(3) Choosing a dealer by a specific limited location: Not all dealers are the same. Dealers have what’s called a CSI which is their Customer Satisfaction Index. These indexes vary from dealer to dealer and could be a good indication of the kind of service you can expect from them. You should probably also check with the Better Business Bureau to see if there have been any complaints against the dealership or the manager of the dealership.
(4)Talking about your possible trade in too soon: Don't mention your trade in until the final price has been firmly set. If you prematurely mention that you are going to trade in your existing vehicle you may find yourself playing their favorite trade-in game. If you do decide to trade in your car make sure that you have it clean to give them the impression that you value your car. If you bring it in dirty then they will have the impression that you do not value your car and want to reduce the trade in value.
(5)Going it alone unprepared. If your not mentally prepared for the negotiating process then you may want to consider an auto brokering service which gives members special pricing through authorized dealers. This definitely eliminates the hassle of the negotiation process.
(6)False sense that the deal has been over and done with. Dealers may think you've completed the deal when they come back and charge you for more stuff like dealer preparation charge, undercarriage treatments, delivery charges etc.. Make sure you have all of the upfront information before you make the final deal.