Most people now ask for shaken martinis, but in fact the times when you shake a drink is when you have juices, heavy creams or cremes. If you shake a martini, chances are it'll look cloudy and not taste any different. It's best to stir it with a wooden or galvanized spoon.

Having said that, the mystique of the phrase "shaken, not stirred" is too classic not to use. :-)