[enabler]

Especially when buying used, don't bring up a trade until after you've settled on a final purchase price. The dealer will hate this and may try to make you feel uncomfortable for not bringing it up before, but that's because they have one less number to play around with. Worst case scenario is that they won't come down any lower, but even huge dealerships need your sale more than you need their vehicle - your options for other fish in the sea are huge. Don't forget this!

Know what your trade is worth and what you're comfortable letting it go for. I find a nice combo between Kelly Blue Book, Edmunds, and N.A.D.A. works well, along with whatever your local ads might tell you. Many dealers want to inspect your trade without you present, whether you're test driving or sitting in some salesperson's office. Then they'll describe all kinds of flaws they noticed, or quote wholesale values inferior to the car's true worth.