Negotiation tactics aside, some useful items are:
- Find out what tax and title should be, assume a $500 "documenting fee" do a notional write up in advance, bring a calculator
- Decide if you are going to get the maintenence plan or entertain it in advance, it can be leverage in the overall deal
- Figure out if you are going to pay for any of their $800 special coatings that are "already on the car" that they will only tell you about at the very end
- Even if you have a good rate from the credit union you may be able to get better or equal from the dealer (manufacturer subsidies). Taking financing from the dealer makes more money for them, so it becomes a lever for you to get the overall price down.
- Take your checkbook (or Amex!) but be ready to bail if they are too stubborn. Leave your contact info and final offer, but the manager already knows you brought your checkbook with you so they will generally come around