It's a tricky thing pricing a car for sale. Price it too low and you cheat yourself out of money to pay for your new car. Price it too high -- either from an emotional attachment or bad research -- and you could be stuck making payments on your new and used cars at the same time. That hurts the pocketbook.
There are two websites that can help you determine a fair value for your car: kbb.com and Edmunds.com. Both will tell you the car's trade-in worth, its private sale value and how much the dealer could expect to sell it for. That last price really demonstrates the absolute highest value you expect to get for the car. No savvy car buyer will ever pay that price to a private individual.
Avoid competitive pricing with newspaper and online classifieds. Some people recommend this, but it can be a waste of time. You have no way of knowing the condition of those cars, regardless of what the ads claim, compared to your vehicle. You're much better off running your car's value through these two competing websites, which are going to be more objective.