Like you, I do my own research and know exactly what model car I want and in what trim. I have my own financing ready to go. I'm not even that picky about price (though I do know what is fair). I'm okay with the sales rep getting a decent commission.
But they still make it a hassle. They try to sneak on things like VIN etching (total rip-off) and chintzy all-weather mats that I can get much cheaper (and better quality) online like with Weathertech. Or those infernal locking nuts that require a special key to get off. I don't plan on parking my car in Harlem and I want the ability to change out a flat tire on a wintry night without having to waste time hunting around for that special socket!
Then you always get the hard sell on the extended warranty (where they have their highest margins) and they try to make you feel stupid for turning it down. I don't need it because I never have plenty of money in my cash accounts - I can pay for an expensive repair every now and then and still come out way ahead of the game.
Why can't they just let a guy buy a car hassle-free! If that ever happened for me, that dealer would have a customer for life.
Anyway good luck and you didn't mention if you had a trade. If you do, take my advice and sell your old car privately or through some other outfit like CarMax (they give a decent price on used cars and cut you a check on the spot). Never trade your car in with the dealer that you are buying your new car from. They are very good at what they do and they will find a way to get over on you with the trade.
Also realize that when you pull into a dealer, once they get you inside, they have somebody doing a walk-around on the car you drove in there looking for blemishes and what not. They are already looking for an angle. Last time I bought a new car, I drove my wife's two-year old car in there and left my 6-year old car at home. I was able to get a decent price negotiated because they thought they were getting a sweet trade-in on the other end. Then I sold my 6-year-old car to CarMax and used the cash to add on to my downpayment.
Finally, never, ever finance your sales taxes and fees into a loan. Pay cash up front for all of that (as part of your downpayment) and only finance what's left on the car. Thirty percent should be the minimum you put down on a car and then no longer than a 60 month loan (48 month is better). That way, you are never upside down on your car and so don't have to worry about silly stuff like "gap insurance."
Excellent advice on the trade-in and not rolling the sales tax into the deal and financing it.