You must actually state that you do not wish to state anything.
http://www.washingtonpost.com/wp-dyn/content/article/2010/06/01/AR2010060101378.html?hpid=topnews
http://www.washingtonpost.com/wp-dyn/content/article/2010/06/01/AR2010060101378.html?hpid=topnews
The ruling comes in a case where a suspect, Van Chester Thompkins, remained mostly silent for a three-hour police interrogation before implicating himself in a Jan. 10, 2000, murder in Southfield, Mich. He appealed his conviction, saying that he invoked his Miranda right to remain silent by remaining silent.
But Justice Anthony Kennedy, writing the decision for the court's conservatives, said that wasn't enough.
"Thompkins did not say that he wanted to remain silent or that he did not want to talk to police," Kennedy said. "Had he made either of these simple, unambiguous statements, he would have invoked his 'right to cut off questioning.' Here he did neither, so he did not invoke his right to remain silent."