“Dress for the job you want, not for the job you have” is a popular employment adage, given to people looking for a job or hoping for a promotion from a present job. John T. Molloy’s book, Dress for Success (1975), popularized the saying “dress for the job you want.” Molloy’s next book, The Woman’s Dress for Success Book (1977), stated:
 
“The rule for all businesswomen is to dress for the job you want, not the job you have.”
 
The saying is still used and has been applied by both men and women. “‘Dress for the job that you want’ is bad advice for aspiring police officers” is a jocular comment on the saying. “Eat for the body you want, not for the body you have” is a similar saying.
   
   
Wikipedia: Dress for Success (book)
Dress for Success is a 1975 book by John T. Molloy about the effect of clothing on a person’s success in business and personal life. It was a bestseller and was followed in 1977 by The Women’s Dress for Success Book. Together, the books popularized the concept of “power dressing.”
 
OCLC WorldCat record
Dress for success
Author: John T Molloy
Publisher: New York : P.H. Wyden, [1975]
Edition/Format:  Book : English
 
3 June 1976, Morning Advocate (Baton Rouge, LA), pg. 18A ad:
Career consultants advise that you “dress for the job you want.”
(Viracle suit from Goudchaux’s—ed.)
 
29 December 1976, Omaha (NE) World-Herald, “Give Albert an Eyeshade; We’ll Overlook Height” by Roger Simon (Chicago Sun-Times Service), pg. 22, col. 5:
“You dress for the job you want.”
(Robert Half, president of an employment agency—ed.)
 
Google Books
The Woman’s Dress for Success Book
By John T. Molloy
Chicago, IL: Follet Pub. Co.
1977
Pg. ?:
The rule for all businesswomen is to dress for the job you want, not the job you have.
 
20 May 1978, San Diego (CA) Union, “Here’s the Nitty-Gritty,” pg. A21, cols. 2-4:
Highlights of Molloy’s The Woman’s Dress for Success book:
(...)
How to avoid staying a secretary; dress for the job you want, not the job you have.
 
Google Books
Women Making It:
Patterns and Profiles of Success
By Ruth Halcomb
New York, NY: Ballantine Books
1981, ©1979
Pg. 146:
“Dress for the job you want to have,” Marge Kinney advised. “If you dress for the job you have now, that’s the image you’ll have and that’s where you’ll stay.”
 
Google Books
Fail-Safe Business Negotiating:
Strategies and tactics for success
By Philip Sperber
Englewood Cliffs, NJ: Prentice-Hall
1983
Pg. 274:
In this section, you will learn what fashions to ignore and how to dress for the job you want, not the job you have.
 
Google Books
April 1985, Cincinnati magazine, pg. 70, cols. 2-3:
Molloy made the point — long accepted by the upwardly mobile — that you should dress for the job you want, rather than the job you have; which generally means dressing one rung up the corporate ladder.
   
Google Books
Careertracking:
26 success shortcuts to the top
By James Calano and Jeff Salzman
New York, NY: Simon and Schuster
1988
Pg. 263:
Dress “up.” Remember the saying, “Dress for the job you want, not for the job you have.”
 
Google Books
A Woman’s Place: Management
By Connie Sitterly and Beth Whitley Duke
Englewood Cliffs, NJ: Prentice Hall
1988
Pg. 112:
Dress for the Job you want, not for the Job you have. Most job applicants know they must dress appropriately to make a good first impression in an interview, but they may forget that they have to maintain a professional image on the job to be considered for a promotion.