The best technique i have found is to average the value from Edmunds.com, kbb.com, and NADA.com, then scan a few local papers to find a few of these cars and formulate the asking price. I'm usually able to come within a few hundred dollars of real value. Keep in mind that dealers tend to use the book that favors the particular transactin at the time. Case in point, I've been told my car is worth $17,500 book and $21,500 book by 2 different dealers, a big difference obviously. You have to do your homework in this area.
Hope I've helped a little. Eli.

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'90 500SL (sold)
17" AMG's