I enjoy making car salespeople squirm =)
Here's what I did the last time I bought a car. ($200 over dealer invoice, tax included out the door)
* I figured out the car I wanted ahead of time. Do not do a test drive with the goal of buying the same day. You will not get a good deal * This also included all the options that I wanted. GPS primarily * I started calling around the last week of month to various dealers within 100 miles. * I offered a price, if they didn't want to play, I gave them my number and rang off. * By the last day of the month, I had the price I wanted in hand and probably could have gotten another $100 off but that was at the point of diminishing returns for me. * Took a 2 hr lunch break to go to the dealership to deal with the paperwork. * 0% financing * Nice and stress free.
Same deal with mattresses. I shaved thousands off the list price.
Thing to keep in mind is that they are always going to win. The goal is not to beat them but to minimize their "win".