How exactly TRADING A CAR works?

I have a 2004 Scion but i still owe 8000 on it and the Blue Book saids its worth 9000 dollars since it stills in sharp shape always garage kept.......anyway how exactly does this works if i wanted to go to the dealer and trade this one for a newer?.......am i better off paying off the car then sell it and wont get what its value is now or what is there a way to do this with out being SHAGGED all over the place by dealerships?....a little help please :o)