I have a fond memory of going with my roommate when he was buying a new car. He had a decent amount to give as a down payment. Armed with the knowledge of how much the dealer pays, he basically walked in, said he'd buy a certain car for a certain amount and not more and handed them his down payment check. The salesman took the offer to his manager. He returned with a counter offer. My roommate didn't budge. After three trips to talk to the manager (each time their offer came down), my roommate remained stuck in his price. On the last trip, their offer was just $500 higher than the desired price. Roommate still held his ground. We watched as the two men were discussing the check in hand. The salesman kept pointing at the check. We couldn't hear what they were saying but we knew it had to be along the lines of "He's going to buy a car. Give him the price he wants." The manager would not come down any lower. We walked out... just $500 away from closing the deal. And the best part... he got a call later that evening asking if he'd still be interested in the car at his desired price.

If you have the time and initiative (and can be willing to walk away without a car) you can get a good deal.