This site sumarizes a report released in October 2007 by the World Cancer Research Fund and the American Institute for Cancer Research.
The report describes the results of five years of study, during which nine teams of scientists reviewed 7,000 studies on diet, exercise, weight and cancer.
They concluded that cancer isn't something that "just happens" -- rather, it's the long-term result of dietary influences and other factors.
Their #1 recommendation for preventing cancer? "Be as lean as possible within the normal range of body weight."
This means having a body mass index of between 21 and 23.