It has been a few years since law school but if I had to use one rule it would be "never talk". If you are being harassed, a good line is "I want to speak to a lawyer", or words to that effect. This is supposed to get them off your back.
The problem is, the U.S. Supreme Court is not real big on the rights of criminal suspects and they have poked all kinds of holes in the Bill of Rights. Another problem is that some cops will lie (they figure that the ends justify the means) and a cop's word is much more likely to be believed over the word of someone accused of a crime.
Also, cops don't always need probable cause to do a search. Another problem is that is is often easy to invent "probable cause". Plenty of people have been nailed for possession of marijuana even after it was found in an illegal search because the cop lies and says he "smelled the odor of burnt marijuana".
It is a common misconception that the cops "have to tell the truth when questioning a suspect". They are permitted to lie and pull all kinds of dirty tricks (good cop, bad cop; give us the goods on your parther and we will do what we can for you, etc.) to get a suspect to give up incriminating information.
You can't really blame the cops for pushing the legal envelope as far as the courts will allow. After all, their job is to make arrests and get convictions.
A couple of other people have mentioned that you should never consent to a search. Good advice. Very often people get stopped on whatever pretense and the officer sort of innocently says "Do you mind if I search your car". Some people think they cannot refuse or think that if they refuse, this will give the cop cause to search. It is true that many cops are going to search no matter what you do, but if they don't have "probable cause" or consent, there is a very good chance that your lawyer could have any evidence that is found thrown out.
This reminds me of something that happened to a friend of mine during our first year of law school. He was driving in the desert near Las Vegas. My friend at that time had long hair, and the local cops apparently assumed that anyone with long hair was up to no good. They pulled him over on some pretense, I think they claimed his brake light was out when it wasn't. The officer spit out real fast "Do you have any weapons, do you mind if we search". My buddy said "No", denying that he had any weapons. Of course, the officer considered this a green light to search his car. If my buddy said "Yes", the officer would consider this an admission that my buddy had weapons. Anyway, the officer starts going through the whole car. In the meantime, my buddy starts thinking about what they've been teaching us in criminal procedure class, and when the officer starts looking into things much too small to contain a weapon, my buddy says "Why are you looking in that, it is obviously too small to contain a weapon." Cop says, "You consented to a search". My buddy responds "I withdraw my consent". If my buddy had been carrying contraband, this would have been a good move. However, the cop made it clear that if my buddy withdrew consent, the cop was going to arrest him on some trumped up charge or otherwise make life miserable. So, wanting to get on his way as soon as possible, my buddy decided to let this asshole go through everything until he finally gave up. Moral of story: never assume that law enforcement plays fair.
All that being said, if somebody was pulled over and foolishly had 100 dbols sitting in plain view and the officer asked what they were, it would probably be a better idea to say they are vitamins in the hope that he won't go farther. If you clam up, he will likely assume that they are contraband and may confiscate them and have them tested = trouble.
Keep in mind that another exception to the general 4th amendment rule against unreasonable searches and seizures is that a cop has the right to pat people down or search the passenger compartment of a car for weapons that might be used against him. This is called a "Terry search". Moral of story, if you must carry something illegal in a car, keep it in the trunk.
Now if you are arrested, then there is no way you should say a word outside the presence of your lawyer, no matter what they may say or offer you.
All right, thats enough rambling.