The overriding theme here is to go in armed with as much information as possible.
These days consumers have a tremendous advantage of being able to find almost anything on the Internet, including invoice costs of the cars and all options.
Do NOT let the dealer fool you into thinking they are losing money by selling a car at invoice either.
If they even mention MSRP or try to use it as a basis for the price you pay, walk away.
If possible, get a quote over the phone and hold them to it. Get the name of the person giving you the quote. If they won't honor it, tell them you're headed over to the BBB office.
Above all else, do not negociate anything but the price of the car first. Some dealerships love to start working finance deals together before you even settle on the price of the car - and this allows them to work all kinds of hidden costs into the finance package that you practically have to be lawyer to detect.
1) Negociate the price of the car.
2) Negociate your trade, if you have one.
3) Negociate the interest rate on your loan, which should be the price of the car minus your trade and down payment.
If you walk in with a $2000 down payment, don't expect the dealer to subtract that amount from the price of the car. Most finance companies won't allow you to finance tax, title, and license fees... so your down payment will actually be what you have left after you pay all that mishmash.
Hope this helps.