As I've only ever bought one car I don't have much experience...but, when buying that I established the going rate online. I read the listings in detail, ignored those that were overpriced, accepted that there would be some minor issues with the car (in my particular case, that the car had failed its safety test a few months before due to two worn tyres, so the dealer only replaced them with used tyres, so I would need to replace two tyres in a few months, and that the car only had one key), made sure that the paperwork was in order (that the dealer had done an HPI check, in the UK this is a full car finance, ownership and accident history check) and bought at the price asked. So didn't have to listen to the dealer waffle on about anything, didn't have to argue over anything, one phone call, one visit for under an hour and car bought. The price was decent versus the book price. At the time I sort of needed the car fairly promptly.
If I were in a situation of not needing a car immediately, I would again establish clearly what I wanted, what I was prepared to pay, make sure I had checked the book prices and was being reasonable, and tell the dealers my requirements and price clearly, offer to pay by their preferred means, tell them to contact me when they could meet my price, walk away if that wasn't offered. If you're prepared to wait a few weeks it should be that a dealer has a car they want to shift to make a monthly/quarterly bonus target, so they will sell to you at your (reasonable) price if you make it clear you're a certain sale.