Right when i'm trading in a car i always go to a dealer that i have no intention of doing business with in the first place!! i push them to a point where they'll let me walk away, leave a contact number wait a day to see if they call back if they do & agree the deal then start asking for extra bits like a full tank of fuel, mats etc, if they agree then buy from there.
If not then i go to another dealer with the previous dealers best offer & try to get them to beat it!!!.
Sounds stupid but it does work, i got £1000 more trade in on my '05 plate ST from £8300 to £9300 + the second dealer matched the offer price of the new ST at £10995 + my options.
Don't take any crap off the sales people push push push, you'll know when they've stopped kidding you!!!.