I ran across this article on Jalopnik today. Basically it's a gameplan on how to beat up your salesman and get the best deal. In actuality, it's a path to failure and making the salesman hate you.
It gives "tips" like "Don't be excited" and "Don't tell the salesperson exactly what you want".
WHY WOULD YOU DO THAT?!
You are trying to get help from me on how to buy a car. Why would you not give me all the information that I need to help you? Doesn't that sound counter-intuitive? And you're buying a car for myself, why wouldn't you be excited? We feed off of your excitement; it causes us to take you seriously.
And really, that's what it boils down to. You have to appear to be interested, otherwise we won't take you seriously. You want to be taken seriously, don't you? Then act interested. Listen to what the salesperson has to say. Hopefully you test drove your salesperson so you know that you're working with someone that you're compatible with. Once you've gotten that perfect salesperson, it's more than ok to let your guard down and tell him or her what you need.
If you want actual good advice on buying a used car, here's my used car buying guide.