Too broad a question for a pat answer.
First though, you will never "beat" a dealer down so far he is hurt on the deal. He knows his limits and won't go below a certain amount.
I try to know values and prices before ever entering into a negotiation. Same rule applies for any item.
My #1 rule is to be willing to walk away from the deal. If you are not able to do that you will pay the price the seller wants.
Rule #2 is show cash. This tells the seller you are serious and able to make the deal and can work in your favor. Plus cash gives a dealer a 4 to 5% margin extra to work with if he doesn't have to pay band credit card fees.