Flaming enthusiasm, backed up by horse sense and persistence, is the quality that most frequently makes for success.

Dale Carnegie (1888-1955) American writer, lecturer

(Attributed)

Widely attributed to Carnegie. The only place I can find this phrase actually used by him "officially" is in Dale Carnegie's Scrapbook (1959) [ed. Dorothy Carnegie], where (four years after his death) the quote is described as "from the writings of Dale Carnegie" but with no further citation.

I was unable to find it online in any books by him. I did find a variant in an essay (1938-09-12) published in newspapers, regarding Alexander de Seversky (possibly one of Carnegie's "5-Minute Biographies" columns):

What are the qualities that make for success? Superior knowledge? Yes, sometimes; but more frequently it is flaming enthusiasm backed up by horse-sense and persistence.

The article also ran in papers on 1946-07-29.

In the 1946 (Vol. 68-70) archive of the Fireman's Fund Record, I did find text reading "... they have flaming enthusiasm backed by horse sense and dauntless courage," but could not confirm the context or whether it was attributed to Carnegie.