I am actually really good at this. We traded in two cars on the 15th for a new Toyota Highlander. We got the SE, almost fully loaded, and got it for under retail value. This is the 4th car we have bought from this dealership, 3rd in two years, so they were working with us, but still.
The finance guy said "I forgot what a pain in my a$$ you are!!" once we started talking numbers :). He said it lovingly though. He tried to give me 2 years of Toyota Care. I just kept saying 5 and it took 3 go-rounds, but we got 5 at no extra cost. We got the SE model for the cost of the one beneath it. So basically we got the leather, sunroof, and navigation for no cost.
Our payments did not decrease nearly as much as we wanted, we are only saving $200 per month with the new car versus the two older cars, but we have a brand new car with no problems. The 2005 Armada and 2009 Prius both needed work on them.
Now we have a 2012 Toyota Camry Hybrid (that my husband drives 90 miles each way to work) and a 2013 Toyota Highlander that I drive around town with the kids. Two new cars that are fully warrantied. I think they know I would have walked without thinking twice and they wanted the deal.
Just do in and be confident. Stand your ground and don't "think" too much in front of them. Be firm and consistent. You'll get a better deal next time.