Possible Clues:

• Work hard on an advertising catchphrase
• Have a swipe at an advertising saying
• "Just Do It," for one
• The 1973 Mets' "Ya Gotta Believe!," e.g
• Advertising tagline
• Commercial line
• Work hard over an advertising catchphrase
• Works as long as marketeer came up with it
• "A diamond is forever," to De Beers
• Campaign phrase