I know more about a car I am going to buy than the salesman that sells it does. That is your first line of defense against those wolves. The second thing you do is call banks and know what intrest rates are at. Knowing what intrest rate you can get is going to go a long way. If you belong to a Credit Union, use them. If you do not belong to one, see if you can join one. Many communities now have credit unions that allow ANYONE the lives or works in the community to join.
Decide on a price you can afford and stick to it. If all you can afford is $15,000, do not shop for $20,000 vehicles.
A family member used to shop by telling a salesman what she wanted and told them to get me your BEST price. No games. Once she got the price, she had it in writing and went to another dealership and said the same thing. When she got the lowest price, they got the sale.
No, "Let me go talk to the manager" BS. You best price and you have one chance. If it is lower then the current one, you go to the top of the list. Highest price, she would get up and walk out.
There are so many ways to negotiate prices. Each person is different.
The internet gives us so many ways to research a vehicle. Knowing what options are available shuts the salesman up before he has a chance to feel you out.
Tom