The best way is to sell the car yourself. Or work out a deal on a new one without telling the salesman you want to trade your old car in. Once you have the price negociated tell him you want to trade your old car in. This method prevents the dealer from inflating the price of the car then offering an attractive price for your trade in (you know what they are offering).
Research the value of your car (and any cars you are interested in) at these sites.
http://kbb.com/
http://www.edmunds.com/