Two lawyers who call themselves the Pot Brothers At Law give advice on what you should do if you are in the marijuana business and get questioned by the police. Their advice is applicable to almost any police questioning situation actually, not just those that are pot-related. (Language advisory)
I believe that the reasoning behind their advice is that when people talk, they tend to say things that are incorrect or mistaken and then they can get charged with lying to law enforcement and that charge can be used to put pressure on them to admit to other things.
(Via Rob Beschizza)