I am the customer car salesmen hate, I go in prepared and will haggle over everything, I never go to buy a car when I have to get a new one, I have had them switch tires off of one car to the one I was buying, free oil changes (didn't pay for any oil changes for 2 years), extra floor mats, etc..... One sales manager made the mistake of saying "you wouldn't walk away over 100 dollars", yes, I would, I got up, told them when you change your mind call me, I didn't even get to the door before they agreed.
I will also mention that the salesmen that have been courteous and nice during the negotiation are the ones I refer people to over and over again and he will get my business again shortly, he was the first car salesman that didn't try to sell me what he thought we needed, but asked what we wanted in a car, we went in thinking we wanted car "A" but walked out of there with car "B" and couldn't be happier.