Keep in mind that while the sales rep is not your friend, neither is he (or she) necessarily your enemy. I want the dealer on any purchase to make a reasonable profit. I want them to be around next year when I need service or to make another purchase.
Do your due diligence before you go in. Be reasonable and PLEASANT. As Mama said, "You get more flies with honey than vinegar." Don't be afraid to say, "Thanks for your time, but we're just too far apart." As others have said, you may not get off the lot, or will get a call the next day.
I get a kick out of all the "tough guys" that claim "that guy didn't make any profit on me." First of all, reread paragraph one. Secondly, any good salesman is doing numerous transactions a month. You're purchasing that major product once every several years? A house three or four times in your life? Do you really think you can beat them at their game? If you turn it into an adversarial transaction, the sales rep will just smile and resolve that, "Screw this guy. I going to nail him somewhere just to prove I'm smarter than he is."