My deal wasn't as great as what you're trying to do- what I did was get them to take out all accessories, undercoating, etc.
But here's what I did if you care: First weekend- test drove two cars, brought a folder and took notes, asked them specific questions about junk I'd read on the website about the cars- you know- how does the boxer engine work vs the standard V engine? I was also critical of the car's gas mileage, design and color choices to put the other guy in defensive mode. People were eating lunch in the dealership and it was making me hungry, but I barked something like "How can you stand to smell everyone else's food?" you know, knocked the guy off his game all the time.
Once I decided which car I wanted, I got the edmunds invoice price- disregard MSRP and all other BS. Don't expect to go too far below the Edmunds price. Technically Subaru gave me $1000 toward the downpayment so I payed $200 above invoice not $800 below, btu $200 above is a good deal. I preset my loan elsewhere and found out their car buying services- went with the Subaru VIP program, but also investigated United Buying Service.
I found a car I wanted from the same dealership chain way out in the middle of nowhere for hundreds cheaper than a car in the big city where I live. I returned to the dealer with an appointment with the same guy who helped me before- he had invested time and wanted a return on his time. Turns out he already had a customer and had his assistant work with me and he'd stop by periodically. Still meant he was off-kilter because he was juggling 2 and 3 people.
I told him we were very interested, but still liked the color and mileage of a Toyota- never cut a sucker and even break- my wife did an act that she wanted the Toyota, but I wanted the Subaru. My wife took our son to the waiting room to play with the toys. I laid my cards on the table- I liked the cheap car I saw in the ad and could they get it for me or should I drive up there to buy it? Of course the guy was able to match the price. Then I said that was great, showed them the VIP pricing program from Subaru- they gave me a song and dance and I said, I'd be interested in them giving me their dealer maintenance package (every other oil change free + loaner car) free instead of reducing the cost of the car further- he agreed and that was a $370 value I planned to buy just for the loaner (I have my reasons, but you may not agree).
When it came time for junk I balanced between- "My wife's not here, let's skip that and ask her later"- that's the "Missing Man" negotiation technique that kept him off-kilter and they gave up on some of those extras. When my wife came back, I put my son on my lap and when the hard sell people came in, I let him slide off my lap and he obligingly ran back to the TV and of course I followed, returning saying we weren't interested in any packages- they were waiting on me and I became "in control." Apparently my wife comiserated with the female sales manager about parenting and daycare costs- tag-teaming and playing up the husband/wife stereotype. They did jerk me around on my trade, but they offered Blue book for the car and I had no problems with that.
I am a terrible negotiator so this took a lot of practice and determination. There was an article on fool.com that was very useful to prepare. Don't expect miracles, but the end of the month is better than the beginning.
Don