Very true. If anybody starts talking numbers and the number they are using is the monthly payment figure, redirect the conversation to the total price of the car. These guys are experts at making you think they're giving you an amazing deal by playing with the numbers to make the monthly payments smaller. In reality, they're just extending the length of the loan out to 72 months, shuffling some money around from one part of the deal to another, and using some slight of hand.
It's also a reasonably good idea to negotiate the price of the car before you tell them you want to trade yours in. They'll ask you right up front if you're trading, and they'll use that as a bargaining chip when negotiating with you. Tell them you haven't decided about trading, and to proceed with the assumption that you won't be trading. Then, when you've negotiated a decent price for the car, tell them you decided you do want to trade in your car after all. The trade-in is just another place they can pull some funny business with the numbers to make it look like they're giving you a great deal.
Oh, and here's another slightly mean trick that I used with my last two car purchases. Look up the dealership's hours. Show up 20 minutes before closing time on a Friday night. They'll all want to go home, be glancing at the clock every 5 minutes since you're keeping them late and they just want to get out since it's the weekend, and are much more likely to cave to your demands just so they can get out of there!