My favorite car site is
www.edmunds.com
You can search the Fair Market Value (FMV) of any new or used vehicle. Once you've decided on the car you want, set a fair "max" price on the vehicle. If you don't want to pay anymore than 'x' amount of dollars for a new car--stick by it. Remember, always start out low and work your way up.
When I bought my Honda new in 1999, the salesman made $150 commission. Not bad for a single gal walking in the dealership, alone.
Oh, and remember--NEVER tell them the pay-off on your existing vehicle. In fact, I wouldn't even mention another vehicle. Once you've settled on a low buying price, you can then settle on a trade-in price.