with a bag of cash that has only the exact amount you are willing to pay for the car.

Then tell the sales guy "I'm here to buy x car. I've done my research, and out the door, fees, taxes, and all, here is what I think I should be paying. I have the cash here needed to pay for that amount, and not a penny more. You've got exactly 30 minutes to talk to your sales manager and figure out if you can make it happen and sign the sales contract. If it goes past 30 minutes, I'm gone and will go find another dealer who can make that happen. The clock starts now."