Always negotiate the deal on your new car first. After you hit the number you want for that, play stupid and say you've been thinking about it and want to trade in your current vehicle. This way you know exactly how much your're paying for the new car and how much you're getting on the trade....and eliminate any funny business such as giving you less of a discount on the new car and more of an allowance on the trade.

I use a similar tactic when I want a lease quote....I negotiate a cash price on the car first to ensure maximum discount off MSRP, then when I'm comfortable with the purchase price say 'I think I may want to lease it' and get a quote based on this discounted number.

Regards...Ray