After the salesperson presents the price, you could respond with: "We've done a lot of research on what this car sells for and we've shopped around a bit. If you can beat this price (here's where you present your best price quote 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.
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 or used car, the key to a successful negotiation is a grasp of the numbers behind the deal.