Know what you want. This means knowing the exact details of the car you want and what you can afford to pay in a cash, finance or lease transaction.
Stick to the points you want to cover. Do not let the saleman throw you off by discussing alternative financing methods, value of your trade-in, etc..
All dealerships are independently owned. They all pay the same invoice cost for the car but there can be significant differences in the price they are willing to sell for. (e.g. a high volume dealer may want to increase its volume to qualify for additional incentives available to them from the manufacturer, whereas a low volume dealer simply wants to move inventory and "Make a Deal").
Know your numbers. You need to know what represents a fair deal. What is the invoice price? What is the dealer holdback? What is the dealer's inventory like?
You can never start too low. You can always increase your offer.
Don't be afraid to walk away. You can always come back and take the deal tomorrow.
Learn to identify the dealers' "best offer". Does the dealer really have no more room to negotiate?
Ask for things that are less costly to the dealer than to you as throw ins. (e.g. oil changes and regular maintenance for the first year free)