My advice would be that you practically cannot negotiate too hard in most dealership purchases. Either new or used, do your research. If there are more than 5 cars of the age and model you want in a 50 mile radius you are in good shape.
Negotiating new is a lot easier than used. Its apples to apples vs apples to oranges.
When I bought my car months back I got a really good internet quote from one dealer. Drove there, and they totally low balled my trade in offer. After some haggling, they bumped my trade in up but still short of my number.
I thanked them for their time and walked out.
Drove to the next dealer, and negotiated a trade in allowance on my car first. It met my number. After that, I pulled out the written quote from dealer #1. Initially, they could not meet that number.
So I thanked them for their time and walked out.
After I got home, both dealerships were calling me and text messaging me to see how they could earn my business.
Eventually, I got the purchase price I wanted and the trade in value at both dealerships within probably $50 dollars of each other. I ended up buying at dealer # 2 because they were a lot more straightforward to work with. Plus far closer to my house than the first place. Traded down a Lexus for a Camry. However, the Camry is a lot more comfortable and doesn't have the luxury car stigma attached to it that I hate.
They key is being willing to walk away from the negotiating table and actually doing it. And you need to be in a position that their desire to sell you a car is more than your need to buy a car. Plus it's good to have at least two dealers you are working with at the same time. Creates a mental advantage for the buyer.
All that being said, I've done not so hot on previous vehicle purchases. Generally because I needed (or wanted probably more like it...) the car they were selling more than they needed to sell it.
I hope to not purchase another vehicle for a long time.