Apparently there is a person who is credited for being the brains behind that slogan and others: Shirley Polykoff
"For Lady Clairol, the cream-and-bleach combination that brought silver and platinum shades to Middle America, [Shirley Polykoff
] wrote, 'Is it true blondes have more fun?' and then, even more memorably, 'If I've only one life, let me live it as a blonde!'"
http://www.ciadvertising.org/studies/st ... olAds.html
http://www.ciadvertising.org/student_ac ... ection.htm
'If I've only one life," exclaimed a model in a 1958 Clairol advertisement, "let me live it as a blonde!"