Be patient. Use your poker face. Leave the Miss/Mr Nice person personality at home.
Be prepared to walk out. In fact, do walk out. The managers will run after you if you really want to make the sale.
If not, try another dealership. Not all dealerships are created equal -- some are good, and some are really bad.
Know your MSRP and dealer's price. Negotiating using the dealer price and NEVER pay MSRP. In fact, I have successfully talked them into giving me the dealer price (they still make money on the car, trust me). You just need to be firm and be prepared to walk out.
There's another thing my friend told me. If you like a car on the lot but it has certain features you don't care about, like sunroof or something, tell them you don't want to pay for that option, but you want the car anyway. Say, the sunroof costs $2000 MSRP, you tell them to knock the price off $2000. They will say, "no way!" And you say, "you have a deal right now if you do that" -- again, be prepared to walk out. Actually WALK OUT. The thing is, it's expensive for the dealership to keep inventory, and they will try to do what they can to get the cars off their lots. So if you know your stuff, they will try to make the sale.
Make sure you read all the fine prints. And if you're leasing or financing, make sure you read and understand everything, and make sure they didn't sneak in numbers and prices that you did not negotiate and come to an agreement on. Sometimes what people didn't realize, for example, is that they agree on a price, say $20,000, and then the lease agreement uses the MSRP (say, $25,000). The dealer will try to con you with that. You have to tell them that's BS. They need to use the negotiated price in everything they type up.
Know your math when you deal with financing or lease agreements. Know all your terms. Don't let the finance guy at the dealership intimidate you.
Don't buy extra services such as rust proof, etc. Those are cash cow for them and utterly overpriced or useless. You may consider an extended warranty if you intend to keep the car beyond the original warranty. Again, negotiate.
If you're trading in, know your blue book values. Look it up on Edmund Used Car prices. If your car is in good shape, you should be able to stick with the blue book value. Or sell your car yourself instead of trading in. Again, that's where the dealership can screw you and make a huge profit.
If you have a friend who knows how to bargain and negotiate (my friend Eddie is EXCELLENT -- he's a dealer himself so he knows all the tricks), bring them with you.