How do you negotiate with used car sales associates and with individuals selling preowned cars?

I'm going to buy a used car soon. How do you negotiate with a used car sales associate for the best price? And how do you negotiate with an individual that's selling his/her used car? Thanks.
How do you negotiate with used car sales associates and with individuals selling preowned cars?
Have an idea what the car is worth first so you know if the selling price is in the ballpark. Don't fall in love with that particular car; there are thousands and thousands of used cars out there. Get a carfax report if you can and be prepared to walk away immediately if you can't come to an agreement or if the seller balks about having your mechanic check it out.