Use KBB and NADA guides as a reference when going into negotiations. I've traded in a couple cars over the years and it never lands on exactly what it's supposed to be worth. The point is make sure the deal overall is a good one. Example:
When I traded in my MSP for the MS6 2 years ago, I got the price of the MS6 down to $16k from their advertised price of $18,500 (stock, 38k miles) and haggled the trade-in of the MSP to $6,500. I only owed $3,800 on the MSP, so I rolled $2,700 of positive equity towards the MS6. This, coupled with a down payment meant I was only actually financing about $12k (after taxes, fees, etc). So it all comes down to how the total deal works out and if it works for your personal scenario. As I've always told friends and family: never roll over negative equity onto a new car. You're only sinking yourself deeper in debt when that happens.
And oh yeah, remember that when trading a car in, aftermarket modifications are either ignored or it devalues it...usually.
