Never talk price in a dealership at all. Inform them that you're just test-driving and all price negotiations will be over email. If they bring up price after you say this, remind them, walk, or ask to work with a different salesperson that will remember your requests better.
When you get to the point of buying send an email to all dealerships in your area: "I want X car, Y year, Z options. Options in excess of Z are nice but I will not pay extra for them. I will be taking bids for final out the door price until (date) and paying on (date) at (time) with a loan from my credit union, so I will not be financing with you. "
Get bids and then send out a second email, "The current low bid is $S. If you can beat that, please email me before (same date as before)." Be honest and not stupid with this step, if you say something made up, everyone will bail as they all know the real bottomline price.
Get the loan paperwork from your credit union and walk into the dealership. Fill out the paperwork and hand it to them, they will work with the credit union to finalize the purchase.