It seems likely that the attribution to Twain is apocryphal. It is not listed as authentic on Twainquotes http://twainquotes.com/, and is not listed at all in either R. Ken Ramussen's The Quotable Mark Twain (1998) or David W. Barber's Quotable Twain (2002) <br class="br">Misattributed

“It is easier to stay out than to get out.”

Mark Twain

“To me [Edgar Allen Poe's] prose is unreadable—like Jane Austin's [sic]. No there is a difference. I could read his prose on salary, but not Jane's. Jane is entirely impossible. It seems a great pity that they allowed her to die a natural death.”

Mark Twain

“When people do not respect us we are sharply offended; yet deep down in his private heart no man much respects himself.”

Mark Twain

“I couldn't bear to think about it; and yet, somehow, I couldn't think about nothing else.”

Mark Twain book Adventures of Huckleberry Finn

Source: The Adventures of Huckleberry Finn

“The radical invents the views. When he has worn them out the conservative adopts them.”

Mark Twain

Mark Twain's Notebook (1935)

“I take my only exercise acting as pallbearer at the funerals of my friends who exercised regularly.”

Mark Twain

Source Undetermined in Everyone's Mark Twain (1972) compiled by Caroline Thomas Harnsberger, p. 161
Disputed

“Against the assault of laughter, nothing can stand.”

Mark Twain

"The Chronicle of Young Satan" (ca. 1897–1900, unfinished), published posthumously in Mark Twain's Mysterious Stranger Manuscripts (1969), ed. William Merriam Gibson ( pp. 165–166 http://books.google.com/books?id=LDvA2xcYZKcC&pg=PA165 in the 2005 paperback printing, ). <br class="br">Source: The Mysterious Stranger and Other Curious Tales <br class="br">Context: Your race, in its poverty, has unquestionably one really effective weapon—laughter. Power, Money, Persuasion, Supplication, Persecution—these can lift at a colossal humbug,—push it a little—crowd it a little—weaken it a little, century by century: but only Laughter can blow it to rags and atoms at a blast. Against the assault of Laughter nothing can stand.

“While the rest of the species is descended from apes, redheads are descended from cats.”

Mark Twain

“There is no sadder sight than a young pessimist, except an old optimist.”

Mark Twain

Variant: There is no sadder sight than a young pessimist, except an old optimist.

“Right is right, and wrong is wrong, and a body ain’t got no business doing wrong when he ain’t ignorant and knows better.”

Mark Twain book Adventures of Huckleberry Finn

Source: The Adventures of Huckleberry Finn

“Nothing that grieves us can be called little: by the eternal laws of proportion a child's loss of a doll and a king's loss of a crown are events of the same size.”

Mark Twain

"Which was the Dream?" (1898)

“In Paris they just simply opened their eyes and stared when we spoke to them in French! We never did succeed in making those idiots understand their own language.”

Mark Twain book The Innocents Abroad

Source: The Innocents Abroad (1869), Ch. 61.
Context: The people of those foreign countries are very, very ignorant. They looked curiously at the costumes we had brought from the wilds of America. They observed that we talked loudly at table sometimes. They noticed that we looked out for expenses and got what we conveniently could out of a franc, and wondered where in the mischief we came from. In Paris they just simply opened their eyes and stared when we spoke to them in French! We never did succeed in making those idiots understand their own language.

“for business reasons, I must preserve the outward signs of sanity.”

Mark Twain