Do your research and put an absolute limit on how much you are willing to spend. Give the salesman that info and stick to it. Standing up to walk out is always an incentive to the salesman to work with you.
Be very picky. Bring up every little problem and blow it out of proportion. It also helps to tell them about similar vehicles you've seen for sell for less. If you tell them your max price the'll try to get every penny of it. Also if you bring in a trade in they could say the will give you so and so for your trade in, but they won't tell you they jacked up the price of the new car to compensate themselves.