Mark Twain on Writing

I conceive that the right way to write a story for boys is to write so that it will not only interest boys but strongly interest any man who has ever been a boy. That immensely enlarges the audience.
- Letter to Fred J. Hall, 10 Aug 1892

To get the right word in the right place is a rare achievement. To condense the diffused light of a page of thought into the luminous flash of a single sentence, is worthy to rank as a prize composition just by itself...Anybody can have ideas--the difficulty is to express them without squandering a quire of paper on an idea that ought to be reduced to one glittering paragraph.
- Letter to Emeline Beach, 10 Feb 1868

Let us guess that whenever we read a sentence & like it, we unconsciously store it away in our model-chamber; & it goes, with the myriad of its fellows, to the building, brick by brick, of the eventual edifice which we call our style.
- Letter to George Bainton, 15 Oct 1888; (first printed in The Art of Authorship: Literary Reminiscences, Methods of Work, and Advice to Young Beginners, Personally Contributed by Leading Authors of the Day. Compiled and Edited by George Bainton. New York: D. Appleton and Company, 1890, pp. 85-88.)

I notice that you use plain, simple language, short words and brief sentences. That is the way to write English - it is the modern way and the best way. Stick to it; don't let fluff and flowers and verbosity creep in. When you catch an adjective, kill it. No, I don't mean utterly, but kill most of them - then the rest will be valuable. They weaken when they are close together. They give strength when they are wide apart. An adjective habit, or a wordy, diffuse, flowery habit, once fastened upon a person, is as hard to get rid of as any other vice.
- Letter to D. W. Bowser, 20 March 1880