10 Brilliant Marketing Ideas From radio to YouTube clips, 10 companies that changed everything.

1. 'A diamond is forever'

Backstory: Diamond prices were sinking fast in 1938, so De Beers mining company enlisted ad agency N.W. Ayer & Son to help reverse the trend. A year later, it launched the "diamond is forever" campaign and brazenly promoted the idea that every marriage required the gift of bling. And plenty of it: It also invented the "two months' salary" spending rule.

Breakthrough: A slogan that transcends the campaign. "It created sentimental meaning for the product that resonated with people," says Michael Cody, communications professor at the University of Southern California, adding that the phrase is so entrenched that some people don't know its commercial origins.