Take it to a dealer and see what they'll give you for it. Odds are they are going to try raping you on the trade-in, but it's always worth a shot. If they get within $800 of private party bluebook, I'd say go for it. That never happens 95% of the time though.
Otherwise, put it up in the autotrader. Check if there are any weekend car fairs by your house, where you can leave the car for sale on weekends. You're going to have a far easier time selling your car than I'm going to have selling my 03 GTS, because far more people are willing to spend $13k on a private party vehicle than close to $20k