[Bring_Back_Shantz]

I'm certainly no expert on this, but make sure you're clear about what you're paying vs what the dealership is getting.

Make it clear that what you are negotiating is the all in price (including tax, transfers, undercoating, blah blah blah).

If you're financing it, make sure you stick to that price. Don't let them go through the whole "What do you want your payment to be" rigmarole, where they dick around with your total price, trade in, financing etc.

Get all of these things in stone first before you start talking about monthly payments.

What will you give me for my trade in?
What financing is available?
What is the price of this truck?

Those are the questions that determine your monthly payment, not the other way around.
And if you're not getting a straight answer from the guy you're dealing with, either ask to speak with someone else, or tell them you're going to go to another dealership where they'll treat you with respect and answer questions directly. You don't have to be a dick about this, but be clear and firm with your expectations.