xy

"English as She Is Taught", The Century, Vol. 33, No. 6, April 1887 http://books.google.com/books?id=EzGgAAAAMAAJ&pg=PA932. A slightly abridged version was reprinted as Introduction http://books.google.com/books?id=CxIuAAAAYAAJ&pg=PR11 to Caroline B. Le Row, English as She Is Taught: Genuine Answers to Some Examination Questions Asked in Our Public Schools (1901)

“Barring that natural expression of villainy which we all have, the man looked honest enough.”

Mark Twain (1835–1910) American author and humorist

"A Mysterious Visit", Buffalo Express, 19 March 1870. Anthologized in Mark Twain's Sketches, New and Old‎ http://books.google.com/books?id=5LcIAAAAQAAJ (1875)

“We said there warn't no home like a raft, after all. Other places do seem so cramped up and smothery, but a raft don't. You feel mighty free and easy and comfortable on a raft.”

Mark Twain book Adventures of Huckleberry Finn

Source: Adventures of Huckleberry Finn (1885), Ch. 18

“Humor must not professedly teach, and it must not professedly preach, but it must do both if it would live forever. By forever, I mean thirty years.”

Mark Twain (1835–1910) American author and humorist

Mark Twain in eruption: hitherto unpublished pages about men and events, 1940, Mark Twain, Bernard Augustine De Voto, Harper & brothers. This appears to be the origin of the variant:
If you would have your work last forever, and by forever I mean fifty years, it must neither overtly preach nor overtly teach, but it must covertly preach and covertly teach.
Attributed to Twain by J. Michael Straczynski in The complete book of scriptwriting, 2002, Writer's Digest Books

“He is now fast rising from affluence to poverty.”

Mark Twain (1835–1910) American author and humorist

"Rev. Henry Ward Beecher's Farm" (1869), anthologized in Mark Twain's Sketches http://books.google.com/books?id=UwcCAAAAQAAJ (1872)

“The humorous story is told gravely; the teller does his best to conceal the fact that he even dimly suspects that there is anything funny about it.”

Mark Twain (1835–1910) American author and humorist

How To Tell A Story (1895)

“Thousands of geniuses live and die undiscovered — either by themselves or by others. But for the Civil War, Lincoln and Grant and Sherman and Sheridan would not have been discovered, nor have risen into notice. … I have touched upon this matter in a small book which I wrote a generation ago and which I have not published as yet — Captain Stormfield's Visit to Heaven. When Stormfield arrived in heaven he … was told that … a shoemaker … was the most prodigious military genius the planet had ever produced.”

Mark Twain (1835–1910) American author and humorist

The Autobiography of Mark Twain (1959 edition, edited by Charles Neider).

“There is no God, no universe, no human race, no earthly life, no heaven, no hell. It is all a Dream, a grotesque and foolish dream. Nothing exists but you. And You are but a Thought — a vagrant Thought, a useless Thought, a homeless Thought, wandering forlorn among the empty eternities.”

Mark Twain book The Mysterious Stranger

The Mysterious Stranger (1916)

“The only way to keep your health is to eat what you don't want, drink what you don't like, and do what you'd druther not.”

Mark Twain book Following the Equator

Pudd'nhead Wilson's New Calendar, Ch. XLIX
Following the Equator (1897)

“Truth is stranger than fiction — to some people, but I am measurably familiar with it.”

Mark Twain book Following the Equator

Pudd'nhead Wilson's New Calendar, Ch. XV
Following the Equator (1897)

“…when the human race is not grotesque it is because it is asleep and losing its opportunity.”

Mark Twain (1835–1910) American author and humorist

Source: Autobiography of Mark Twain, Vol. 2 (2013), p. 127

“Wagner's music is better than it sounds.”

Mark Twain (1835–1910) American author and humorist

Actually by Bill Nye, possibly confused due to Nye quoting Twain in More Tramps Abroad, 1897. (See also autobiography, vol. 1, p. 288.)
Misattributed

“I have seen Chinamen abused and maltreated in all the mean, cowardly ways possible to the invention of a degraded nature, but I never saw a policeman interfere in the matter and I never saw a Chinaman righted in a court of justice for wrongs thus done him.”

Mark Twain (1835–1910) American author and humorist

"The Treaty With China", article in The New York Tribune, 1868-08-04. Quoted in Mark Twain's Letters, volume ii, p. 239 https://books.google.com/books?id=EWvU21-vV8EC&pg=PA239&lpg=PA239&dq=%22I+have+seen+Chinamen+abused+and+maltreated+in+all+the+mean,+cowardly+ways+possible+to+the+invention+of+a+degraded+nature.%22&source=bl&ots=-MSeb52ibq&sig=7EJ2Hkgp58wiQNoBmWysiM5YcIQ&hl=en&sa=X&ved=0ahUKEwjMxPKKvbTMAhUM4mMKHbICCt0Q6AEIHTAA#v=onepage&q=%22I%20have%20seen%20Chinamen%20abused%20and%20maltreated%20in%20all%20the%20mean%2C%20cowardly%20ways%20possible%20to%20the%20invention%20of%20a%20degraded%20nature.%22&f=false

“Before taking final leave of me, my instructor inquired concerning my physical strength, and I was able to inform him that I hadn't any.”

Mark Twain (1835–1910) American author and humorist

"Taming the Bicycle" (1917)