Two things I recall from my last dealer interaction - six years ago: negotiate an "out the door" price, and the deal ain't done until you drive home.
"Out the door price" means the total amount you pay, including all fees and charges. The dealer splits their profit into markup and various fees. A dealer may give a nice price, and then tell you there is a $500 "documentation fee" or "advertising fee", explaining that it is non-negotiable. (Often under the guise that the dealership has no control over the fee amount. For example, this is what Ford charges the dealership to process documentation, or this is a fee to cover advertising for all local Ford dealerships, or some other story.) Often this "fee" does not get mentioned until the very end. My stance was, I don't care what you call the markup - and I understand the dealer is not a charity. But tell me the total amount that I will pay to drive the car home.
Secondly, once I finalized my negotiations, I was taken to another office. For one hour, a kind gentleman tried to sell me paint sealant, interior protection, extended warranty, and many other "upgrades", most of which (not all) I likened to snake oil. I did not buy any.
Final point, sometimes it helps to go with dealer financing, even if you don't need it. Make sure it is a low rate and does not have an early payment penalty. By getting it, you may be "throwing the dealer a bone" (they get a commission for the sale), and then you can do what you like with the loan (pay or keep it).