Don't forget, when you trade cars, they are not giving you anything.They allow you so much off the top, but you are far better off to sell your old car and offer the dealer to reduce the amount you got from the sale.Show them money and they will deal.
Cash, no trade will give you a better deal. Ask yourself, what will he do with my old car? More than likely he will scrap it because he can't resell a used up clunker.

Of course, this mostly applies to old cars with little life left in them.
So, money talks, bullshit walks. Keep this in mind when the dealer asks, "Do you have something to trade"? He doesn't want you to know the true value of either car.In this case he must add in the cost of getting rid of your old trade in.