I did talk to someone from New Century, if that's the dealership you are talking about. The internet salesman was offering me something like 5 to 6 % off, which I had no interest in. I went to a family owned BMW in San Diego, the place was dirty, bathroom looked like one from gas station, their best price was like 5% ( I know it was their best price because I told them I was ready to buy on that day and they still let me walked out.)
I am no expert. I guess it also requires some luck. If the dealer needs your business at the time when you come in, they try hard to please you.
Bring the "what others are paying" distribution graph with you to dealership. Ask if those 10% customers are getting 8% off MSRP, why can't they offer you that.