"Them" who? Little different private party vs. car lots.
I go in assuming AT LEAST 10% off anyone's asking price.
Never EVER believe that their first deal is their best, especially on trade in value offers. It's not.
Realize that salesmen have all the time in the world. You're the uncomfortable one in their environment. They're in no hurry. You need to be the same way.
Ask to see every fee they try to charge you, usually around or above $250. They are negotiable too.
If you know you can't talk them down anymore, go after the free oil changes or free car washes, or tank of gas.
Don't expect to talk anyone down 4 grand on a 7 grand car.
Pay part on credit card if you get rewards and in the position to do so.
Point out everything that looks like it needs repaired or could cost you cash after the test drive. Expect it fixed or more cash taken off the price.
Remember it is not a friendly game of poker, it's business. You can't be afraid or nervous about offending someone with an offer. What's the worst they can do, say no or make a counter offer?
Some salesmen are great at their job, but you're still the buyer with the cash.