Mark Twain, who read widely, was passionately interested in the problems of style; the mark of the strictest literary sensibility is everywhere to be found in the prose of Huckleberry Finn . . . He is the master of the style that escapes the fixity of the printed page, that sounds in our ears with the immediacy of the heard voice, the very voice of unpretentious truth.
(Lionel Trilling, "Mark Twain's Colloquial Prose Style", from The Liberal Imagination, 1950).

As his legions of readers are well aware, Mark Twain gloried in expressing himself through language. And throughout his life, this master stylist had a great deal to say about the art and craft of writing.

• The Best Time to Start Writing
  The time to begin writing an article is when you have finished it to your satisfaction. By that time you begin to clearly and logically perceive what it is that you really want to say.
  (Mark Twain's Notebook, 1902-1903)
• Getting the Right Word in the Right Place
  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, February 1868)
• Good Grammar
  I like the exact word, and clarity of statement, and here and there a touch of good grammar for picturesqueness.
  (The Autobiography of Mark Twain, 1924)

• The Rules of Grammar
  I am almost sure by witness of my ear, but cannot be positive, for I know grammar by ear only, not by note, not by the rules. A generation ago I knew the rules--knew them by heart, word for word, though not their meanings--and I still know one of them: the one which says--but never mind, it will come back to me presently.
  (The Autobiography of Mark Twain, 1924)
• The Subjunctive
  Damn the subjunctive. It brings all our writers to shame.
  (Notebooks, 1935)
• Style and Matter
  Great books are weighed and measured by their style and matter, and not the trimmings and shadings of their grammar.
  (Speech at the Annual Reunion of the Army and Navy Club of Connecticut, April 1887)
• The Queen's English
  There is no such thing as the Queen's English. The property has gone into the hands of a joint stock company and we own the bulk of the shares.
  (Pudd'nhead Wilson, 1894)
• Writers Who Favor Foreign Phrases
  They know a word here and there, of a foreign language, and these they are continually peppering into their literature, with a pretense of knowing that language--what excuse can they offer? The foreign words and phrases that they use have their exact equivalent in a nobler language--English; yet they think they "adorn their page" when they say Strasse for street, and Bahnhof for railway station, and so on--flaunting these fluttering rags of poverty in the reader's face and imagining he will be ass enough to take them for the sign of untold riches held in reserve.
  (A Tramp Abroad, 1880)
• Revising
  You need not expect to get your book right the first time. Go to work and revamp or rewrite it. God only exhibits his thunder and lightning at intervals, and so they always command attention. These are God's adjectives. You thunder and lightning too much; the reader ceases to get under the bed, by and by.
  (Letter to Orion Clemens, March 1878)
• Student Compositions
  The prime feature of the evening was in order, now--original "compositions" by the young ladies. Each in her turn stepped forward to the edge of the platform, cleared her throat, held up her manuscript (tied with dainty ribbon), and proceeded to read, with labored attention to "expression" and punctuation. The themes were the same that had been illuminated upon similar occasions by their mothers before them, their grandmothers, and doubtless all their ancestors in the female line clear back to the Crusades. "Friendship" was one; "Memories of Other Days"; "Religion in History"; "Dream Land"; "The Advantages of Culture"; "Forms of Political Government Compared and Contrasted"; "Melancholy"; "Filial Love"; "Heart Longings," etc., etc.
  
  A prevalent feature in these compositions was a nursed and petted melancholy; another was a wasteful and opulent gush of "fine language"; another was a tendency to lug in by the ears particularly prized words and phrases until they were worn entirely out; and a peculiarity that conspicuously marked and marred them was the inveterate and intolerable sermon that wagged its crippled tail at the end of each and every one of them. No matter what the subject might be, a brain-racking effort was made to squirm it into some aspect or other that the moral and religious mind could contemplate with edification. The glaring insincerity of these sermons was not sufficient to compass the banishment of the fashion from the schools, and it is not sufficient to-day; it never will be sufficient while the world stands, perhaps. There is no school in all our land where the young ladies do not feel obliged to close their compositions with a sermon; and you will find that the sermon of the most frivolous and the least religious girl in the school is always the longest and the most relentlessly pious.
  (Chapter 21, The Adventures of Tom Sawyer, 1876)
• Adjectives
  As to the Adjective: when in doubt, strike it out.
  (Pudd'nhead Wilson, 1894)
• Verbosity
  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, March 1880)

For more tips and observations from professional writers, see Writers on Writing: Advice From the Pros.