What to say at the dealership
Three sentences win most of the money. Say them in this order, before anyone mentions a monthly payment.
Never negotiate the monthly payment. It's the answer, not the question.
Then protect the win
Leverage to apply, and the places a won deal quietly un-wins itself.
Red flags
If you hear it, slow down.
Take it with you
A one-page sheet with your target numbers, the math behind the payment, every line to say, and all the red flags. Save it as a PDF or print it.