A close paraphrase of this (beginning "Do not use...") is repeated in a commencement address at Cloves Hall, 27 April 2007, as reprinted in Armageddon in Retrospect
A Man Without a Country (2005)

“Robert Kennedy, whose summer home is eight miles away from the home I live in all year round, was shot two nights ago. He died last night. So it goes.
Martin Luther King was shot a month ago. He died, too. So it goes.
And everyday my government gives me a count of corpses created by the military service in Vietnam. So it goes.
My father died many years ago now — of natural causes. So it goes. He was a sweet man. He was a gun nut, too. He left me his guns. They rust.”

Kurt Vonnegut book Slaughterhouse-Five

Slaughterhouse-Five (1969)

“Humor is an almost physiological response to fear.”

Kurt Vonnegut book A Man Without a Country

A Man Without a Country (2005)

“I wish that people who are conventionally supposed to love each other would say to each other, when they fight, "Please — a little less love, and a little more common decency."”

Kurt Vonnegut book Slapstick

Prologue
Slapstick (1976)

“Just because some of us can read and write and do a little math, that doesn't mean we deserve to conquer the Universe.”

Kurt Vonnegut book Hocus Pocus

Hocus Pocus (1990)

“There is no reason why good cannot triumph as often as evil. The triumph of anything is a matter of organization. If there are such things as angels, I hope that they are organized along the lines of the Mafia.”

Kurt Vonnegut book The Sirens of Titan

Source: The Sirens of Titan (1959), Chapter 7 “Victory” (p. 165; epigram)

“Many people need desperately to receive this message: "I feel and think much as you do, care about many of the things you care about, although most people don't care about them. You are not alone."”

Kurt Vonnegut book Timequake

Timequake (1997)

“1. Find a subject you care about.
2. Do not ramble, though.
3. Keep it simple.
4. Have the guts to cut.
5. Sound like yourself.
6. Say what you mean to say.
7. Pity the readers.”

Kurt Vonnegut

As quoted in Science Fictionisms (1995), compiled by William Rotsler
Various interviews

“So let’s give another big tax cut to the super-rich. That’ll teach bin Laden a lesson he won’t soon forget.”

Kurt Vonnegut

Cold Turkey (2004)

“Humor is a way of holding off how awful life can be.”

Kurt Vonnegut book A Man Without a Country

A Man Without a Country (2005)

“profanity and obscenity entitle people who don't want unpleasant information to close their ears and eyes to you.”

Kurt Vonnegut book Hocus Pocus

Hocus Pocus (1990)

“I have this theory about why men kill each other and break things. … Never mind. It's a dumb theory. I was going to say it was all sexual … but everything is sexual … but alcohol.”

Kurt Vonnegut Happy Birthday, Wanda June

"Mildred"
Happy Birthday, Wanda June (1970)

“George W. Bush has gathered around him upper-crust C-students who know no history or geography.”

Kurt Vonnegut book A Man Without a Country

A Man Without a Country (2005)

“If I should ever die, God forbid, let this be my epitaph:
THE ONLY PROOF HE NEEDED
FOR THE EXISTENCE OF GOD
WAS MUSIC”

Kurt Vonnegut

As quoted in "Vonnegut's Blues For America" Sunday Herald (7 January 2006)
Various interviews

““You finally fell in love, I see,” said Salo.
“Only an Earthling year ago,” said Constant. “It took us that long to realize that a purpose of human life, no matter who is controlling it, is to love whoever is around to be loved.”

Kurt Vonnegut book The Sirens of Titan

Epilogue “Reunion with Stony” (p. 313)
The Sirens of Titan (1959)