Substitute “damn” every time you’re inclined to write “very”; your editor will delete it and the writing will be just as it should be.

Mark Twain (1835-1910) American writer [pseud. of Samuel Clemens]
(Spurious)

Written as such, I was unable to find any online book reference with the expression prior to 2001, except for a possible 1982 second-hand mention in a Forbes Magazine letter column. Further research (see QI link below) finds the first mention of something substantially like this in 1935, recollecting a similar statement by newspaper editor William Allen White several years before. All post-date Twain's death.For more details, and other variants of this quotation, see Advice Origin: Substitute ‘Damn’ Every Time You’re Inclined to Write ‘Very’ – Quote Investigator®.