Being able to walk is a significant strength.
Never hand over the keys to your ride until the deal is set in stone.
If anyone tries anything after the price is agreed and a handshake is made (like when they pass you off to the financing department and make up bullshit "admin fees") walk immediately.
You can't hurt a car salesman's feelings, he doesn't have any. Not about you, anyway. You're just another $1000 commission sucka on the lot.
Negotiate from a position of strength. Know the invoice, holdbacks, and other promotions. Know the options, what you want and what you don't want. Make sure you can actually afford the car. Better if you can pay cash (but don't tell the guy it's a cash sale, or talk about trades until the price is written in stone). If asked what monthly payment, stick firm to your need to have a firm "out the door, no other extras" price. We can talk financing later.
If you are going to finance, know your options. If your credit sucks, it sucks to be you and you are at a disadvantage, especially if your bank won't finance you. If it's good, make sure you have all the options weighed before you meet "the finance guy".
Personally, if the guy does the "I need to talk to my manager" routine, I tell him he has 5 minutes, and then I'm walking. then I start a stopwatch. And I will walk. They always call back, or their internet sales guys do. I'll let them play their charade a little bit, but have other things to do rather than sit in uncomfortable silence because I know the shitbag probably has the speakerphone on, and is hoping my wife will say something he can use for leverage.
On that subject, if your wife isn't hard-nosed uber-bitch (mine's all soft and fluffy and emotional, and I kinda like 'em that way) tell her before you walk into the dealership to STFU. If she won't STFU, leave her at home. Seriously. Set expectations with her that you probably won't walk away with a new car today - and we know how hard women find it not to piss $300 on fucking useless shoes up the wall, so watch out when she wants a $50K SUV and whines that paying for all the options she doesn't need, or want, because it's "blue" is OK and it's only an extra $40 a month, or some "nominal" (in her mind) amount of cash.
That's all I got. I recognize that everyone needs to make money, so I don't want to fuck anyone, but I know the dealer won't let himself be fucked, so I can be a nasty, miserable, old cunt. I don't "need" a new car. I don't really "want" a new car, and once I've nailed you down on price, I'm going to fuck your finance guy in the ass by writing a check anyway.
Alternatively the USAA or Costco programs will hook you up with the fleet or internet sales departments and you'll simply pay an agreed $x over invoice no matter what you spec, and you're out the door, save for the finance and trade.
Dave Ramsey probably wouldn't like my potty mouth, but he got me to a reasonable position of financial strength. Every so often it's nice to be reminded where you are.