Whatever you might believe, your state of mind is important. You are entering the battlefield. The car dealer is in his comfort zone. You want to be in tip-top condition before you face him. Make sure you have eaten, slept well the night before, and are not sick or otherwise stressed. You want to exude confidence in what you're doing. A car dealer will scent hesitation and do their best to take advantage of it. You can't change where the negotiations are held, but you can deny them this advantage.
Do your homework on the car you want to buy, what it will cost, and have an offer fixed in your mind before you even walk into the showroom. A phone quote on a car will probably be lower than a quote in person; compare the difference. Call all the dealerships in the area first to see how many of them carry the car you want. The more dealerships that carry the car you want, the more walk away leverage you have.
Stay polite at all times. You're not trying to make a scene. You're just playing hardball.
Is negotiating with a car dealer a game of patience or a game of speed? There are different tactics that involve both. Use the one that you are most comfortable with. If you are playing a fast game, make it clear that you are only going to stay 30 minutes or an otherwise short time period. If you're going to play a patience game, then make them sweat.
Negotiate from wholesale cost upward; not MSRP downward. Disregard any invoice prices the dealer might try to show you. They print the invoices, of course the invoice numbers are what they say they are.
Don't back down from your offer especially if it's reasonable (wholesale cost plus 5 percent or so). If a dealer tries to bluff you, hold up your hand to break their stride, and explain how you arrived at the numbers you did. If they say you're wrong, have them check online right there and then. They can ridicule you, but if they end up selling you the car, it means you were right all along. Do not be afraid to walk away from a deal if it simply isn't right.
When car dealers start mentioning payments, stop them and ask clearly for the total cost once everything is settled. Dealers love the word payment because it allows them to quote cars in terms of monthly payment, leaving the purchase price ambiguous until the papers are signed.
Getting a good deal in both trade-ins and buying a new car requires some strategy. Hide the fact that you intend on trading in an existing vehicle when negotiating. Lie about not having a trade-in, and then say you changed your mind later on. They'll lie to you, so don't feel too bad about it.
In rare occasions, usually limited edition cars with high demand, the car dealers can literally sell at MSRP and turn away anyone who tries to negotiate. This is the odd case, and if you're seeking a car that is in such high demand, you probably knew about it beforehand.