Briefly outline your product and what you can do for his or her company. In many cases, the top person, or an assistant, will make a note on the letter and direct it back down through the ranks. That notation carries weight. There are, of course, other ways to make the initial contact. Voice mail is a terrific tool for getting through to the person in charge.
Most executives have a direct line.
Ask to be put through by the receptionist. Try phoning during the lunch hour, too. Even if the top person signs off on your project, middle managers will sometimes exercise their powerful veto power. And at times they'll try to sabotage you. After one project I had nurtured nearly to completion died a slow death from lack of middle management support.