You are not trying to make a new friend when you are buying a car. Be gracious, build a rapport, but do not get too friendly. On the other hand car dealers are not going to put up with rudeness.
Do not be afraid to ask for a discount. Be confident, and ask for the dealer to sympathize with you. Say something like: "This is a great car, but my wife will kill me if I spend what you're asking.
Whether or not your wife would be upset is irrelevant. This is called appealing to someone with greater authority than you. You can stay on good terms with the dealer, but still push for a good deal. It makes the seller more inclined to move on price.
Do not accept the first offer after your plea for sympathy. No matter the offer say something like, "hmmmm, I still do not think my wife would buy that." Or, "You can not do better than that?"
Silence is a powerful negotiating tool. After asking him to improve his offer always resist the urge to say something else. Silence can be painful, but its necessary to get a good deal on your car.
Do your research on car prices in the area. Always ask "Will you match your competitor's prices?" And be prepared to say "these guys are selling for X price."
Ask the car dealer for added value. Once you've exhausted other potential discount options, ask for add on to sweeten the deal.
These tips apply to shopping for new and used cars. Stick to the plan and you will get a great deal. Good luck!