They will lie or provoke you into doing something, even worse, one of the cops I know had a story about how someone said, "no, you cannot search my vehicle." The cop then asked if he wanted to be arrested for resisting arrest, and the guy gave in because he has no idea what his rights really are. The cop who was telling me seemed pretty smug about it, tricking someone into searching his car...
The point being, it's very hard not to get yourself into a pickle of the police officer really wants you. Most officers have somewhere between 2 - 4 years of formal education, most of which is about how to trick people, not about law. If the police officer suspects you're at fault for anything (or even is just having a bad day and doesn't like your look) he will arrest you.
That being said, always say, "I'm not refusing, but you may not search my car (or persons)" and don't go with them unless you are arrested. Both require them to get a warrant, something they wont do unless they need to. If your pulled over and asked why, always say "no." (fifth amendment right and all).
Finally, always, always, always be super nice and professional. Act innocent and you wont make the officer angry.