Get the price in writing... That's what I did and I've got their hand written price sheet along with my final price and finance numbers on their papers. It has my trade-in value, price of the vehicle, incentives, etc.
Whenever you have things in writing, your butt is covered if someone tries to screw you. However, if they tried to do that to me, I'd also second guess trying to get a vehicle from them, let alone getting warranty work done there.
The best part about negotiating is you're not bound to anything. Sit down, talk it over, haggle, and if you're not pleased walk away. Actually, try walking away, they usually lower their prices faster. I told them the price range I wanted and they kept hovering over it. When I asked them how long the prices were good for and was about to walk away they magically came within the price I wanted to pay. Go figure.
'13 'Stang - V6 Base GHIG - Ordered: 5/31 - VIN Issued: 6/7 - Window Sticker: 6/19 - Build Date: 6/25 - ETA: 7/6 - Received: 7/3
'00 Grand Prix - L67 Swap, 3.25" Pulley, Cam, Headers, etc.