Here are some quotable quotes by Mark Twain:

Adversity:

 By trying we can easily learn to endure adversity--another man's I mean - Following the Equator.

Age:

 Wrinkles should merely indicate where the smiles have been. - Following the Equator

Beauty:  There are women who have an indefinable charm in their faces which makes them beautiful to their intimates, but a cold stranger who tried to reason the matter out and find this beauty would fail. - A Tramp Abroad

Camel:

 When he is down on all his knees, flat on his breast to receive his load, he looks something like a goose swimming; and when he is upright he looks like an ostrich with an extra set of legs. Camels are not beautiful, and their long under lip gives them an exceedingly "gallus" expression. They have immense, flat, forked cushions of feet, that make a track in the dust like a pie with a slice cut out of it. They are not particular about their diet. They would eat a tombstone if they could bite it. A thistle grows about here which has needles on it that would pierce through leather, I think; if one touches you, you can find relief in nothing but profanity. The camels eat these. They show by their actions that they enjoy them. I suppose it would be a real treat to a camel to have a keg of nails for supper. - Innocents Abroad

Dictionary:

 Mark Twain quaintly writes (London, Feb. 6th) of the Edition for 1900: "In my experience I have found that one can do without principles, but not without the Funk & Wagnalls Standard Dictionary." - advertisement in New York Sun, April 14, 1900, p. 7.

Failure:

 It is not in the least likely that any life has ever been lived which was not a failure in the secret judgment of the person who lived it. - Mark Twain's Notebook

Genius:

 It is impossible that a genius--at least a literary genius--can ever be discovered by his intimates; they are so close to him that he is out of focus to them and they can't get at his proportions; they can't perceive that there is any considerable difference between his bulk and their own. - Autobiography of Mark Twain, (reference to Jim Gillis)

Hardships:

 It is poison--rank poison to knuckle down to care and hardships. They must come to us all, albeit in different shapes--and we may not escape them--it is not possible--but we may swindle them out of half of their puissance with a stiff upper lip. - quoted in Mark Twain's Letters to Will Bowen

Ideas:  There is no such thing as a new idea. It is impossible. We simply take a lot of old ideas and put them into a sort of mental kaleidoscope. We give them a turn and they make new and curious combinations. We keep on turning and making new combinations indefinitely; but they are the same old pieces of colored glass that have been in use through all the ages. - Mark Twain, a Biography

Ideals:

 It is at our mother's knee that we acquire our noblest and truest and highest ideals, but there is seldom any money in them. - Mark Twain, a Biography

Keller, Helen:

 I am filled with the wonder of her knowledge, acquired because shut out from all distractions. If I could have been deaf, dumb, and blind I also might have arrived at something. - Mark Twain's Speeches

Language:

 But language is a treacherous thing, a most unsure vehicle, and it can seldom arrange descriptive words in such a way that they will not inflate the facts--by help of the reader's imagination, which is always ready to take a hand and work for nothing, and do the bulk of it at that. - Following the Equator

Love:

 Love seems the swiftest, but it is the slowest of all growths. No man or woman really knows what perfect love is until they have been married a quarter of a century. - Notebook, 1894

Obedience:

 Always obey your parents, when they are present. Most parents think they know more than you do; and you can generally make more by humoring that superstition than you can by acting on your own better judgement. - Advice to Youth, 4/15/1882

Pessimism:

 It is easy to find fault, if one has that disposition. There was once a man who, not being able to find any other fault with his coal, complained that there were too many prehistoric toads in it. - Pudd'nhead Wilson

Radical:

 The radical of one century is the conservative of the next. The radical invents the views. When he has worn them out, the conservative adopts them. - Notebook, 1898 School:

 Every time you stop a school, you will have to build a jail. What you gain at one end you lose at the other. It's like feeding a dog on his own tail. It won't fatten the dog. - Speech, 11/23/1900

Talent:

 We are always more anxious to be distinguished for a talent which we do not possess, than to be praised for the fifteen which we do possess. - Mark Twain's Autobiography; More Maxims of Mark, Johnson, 1927

Vanity:

 We do not deal much in facts when we are contemplating ourselves. - "Does the Race of Man Love a Lord?"

War:

 An inglorious peace is better than a dishonorable war. - "Glances at History," 1906

Youth:

 The elastic heart of youth cannot be compressed into one constrained shape long at a time. - The Adventures of Tom Sawyer