You simply have to be able say ‘no, thanks at that price’ at least once to the dealer. This gives them a strong message that you are serious about your research.
You should also bring a piece of paper to the dealership and make sure you do all the math of the finance calculations yourself. The point is not that they will do the math wrong. The point is you will see exactly how the deal is structured. Do not be afraid to take the time to do this or look like a fool for mapping out your car deal in the dealership.