1. Locate the closest dealer to you that has the unit you want (understanding that this could be several states away of course)
2. Negotiate a "ballpark price" for the unit to make sure it's near what you can/want to pay
3. Then bring up your tradein...keeping in mind that most of the time the more they give you for your tradein the more you're going to pay for their unit.
The reason I brought up negotiating a "ballpark price" BEFORE mentionting your tradein is because it gives you a place to start when trying to negotiate the price for theirs and tradein value on yours.
Dealerships will give you a ballpark value on your tradein over the phone if they want your business. Keep in mind that it ultimately will depend on the condition of your unit and the dealership actually seeing it and inspecting it, but at least by getting that ballpark tradein value you'll know whether it's worth your time to take that next step with them.