After the salesperson presents the price, you could respond with: "We've done a lot of research on the market value of this vehicle, and we have a good idea of what this car sells for because we've shopped around a bit. If you can beat this price (here's where you present your best price quote printout from another dealership) we will have a deal."
A response like this accomplishes a couple things. First, it lets the salesperson know that you're an informed buyer. Your goal is to justify the drop in price rather than present an offer without context. Second, the salesperson knows there are offers to beat. So there is a good chance the salesperson is going to jump to the low end of the dealership's pricing structure. This works for both new cars and used cars.
If you are shopping for a used car, where apples-to-apples comparisons aren't possible, your goal is to make an offer that is as low as possible but still in the ballpark.
For a new car or used car, the key to a successful negotiation is a grasp of the numbers behind the deal.