Negotiating tactics require you to put the pressure on the other guy -- the dealer -- to come up with the best price, rather than given him the upper hand to pressure you. By following a few rules, especially in economic conditions that put the buyer is in control, you could save thousands on your next car purchase.
Be an informed consumer. Check out price information websites (find links in the References section) and read reviews, find out what price others are paying, and read about what other consumers say are the weaknesses of the car you want to purchase.
Send out multiple requests to dealers by email and on the web regarding on the model car that you want to buy. While this is ideally suited to new cars where every car is in the same condition, it is also useful for comparing similar used cars.
Always ask for the "out-the-door" price because you want to make sure you are comparing apples to apples. Many dealerships will tack on additional fees if you do not ask for their "OTD" pricing.
Most dealers have the ability to quote different prices over email and the internet than they can in person. The internet consumer is more informed generally, and knows the rebates, incentives and dealer pricing currently in place.
Once you have multiple quotes, know that they can get much lower. Take the lowest quote that you have been given and email every OTHER dealer that responded and ask if they can beat your lowest quote. Do not call the dealer with the lowest quoted price yet.
Once you have narrowed down the group to those that were able to beat the price that was originally the lowest, call that dealer and tell them that they need to beat your new "lowest" price. Since this was the dealer that was the most aggressive the first time around, there is a good chance that they will continue to be the most aggressive. Don't show up in person until you feel you have a price that is reasonable. You give up significant leverage once you show up in the dealership.
Always put the pressure on the dealer once you arrive. Have an important appointment that you need to leave for, have a friend / spouse call you with an "urgent issue", etc. They will be anxious to close the deal before you leave, and you can usually get them to take at least another $100 off.
People often feel that negotiating over $100 on a large purchase is a waste of time, but think how excited you would be if you found $100 on the sidewalk walking home. Always look at it with that perspective.