Great quotes!

How does one come up with witty comebacks or quotable quotes like "Only Robinson Crusoe had everything done by Friday" (I had used this gem recently)?

Anyway, enjoy the ones here:

The cardiologist's diet: If it tastes good ... spit it out. -- Unknown

“I once had a rose named after me and I was very flattered. But I was
not pleased to read the description in the catalogue: 
'No good in a bed, but fine up against a wall’ ". -- Eleanor Roosevelt

“The secret of a good sermon is to have a good beginning, and a good
ending; and have the two as close together as possible.” -- George Burns

“Santa Claus has the right idea ... visit people only once a year.” -- Victor Borge

“Be careful about reading health books. You may die of a misprint.” -- Mark Twain

“What would men be without women? Scarce, sir, mighty scarce.” -- Mark Twain

“My wife is a sex object. Every time I ask for sex, she objects.” -- Les Dawson

“By all means marry. If you get a good wife, you'll become happy; if
you  get a bad one, you'll become a philosopher" -- Socrates

“I was married by a judge. I should have asked for a jury.” -- Groucho Marx

“My wife has a slight impediment in her speech. Every now and then she
stops to breathe.” -- Jimmy Durante

“The male is a domestic animal which, if treated with firmness and
 kindness, can be trained to do most things.” -- Jilly Cooper

“I never hated a man enough to give his diamonds back.”
“I was always a good housekeeper.  Whenever I divorced I always kept
 the house.” -- Zsa Zsa Gabor

“Only Irish coffee provides, in a single glass, all four essential
food  groups: alcohol, caffeine, sugar, and fat.” -- Alex Levine

“My luck is so bad that if I bought a cemetery, people would stop
 dying.” -- Ed Furgol

“Money can't buy you happiness, but it does bring you a more pleasant form of misery.” -- Spike Milligan

“I am opposed to millionaires, but it would be dangerous to offer me
the  position.” -- Mark Twain

"Youth would be an ideal state if it came a little later in life.” -- Herbert Henry Asquith

“I don't feel old. I don't feel anything until noon. Then it's time
for my  nap.” -- Bob Hope

“A woman drove me to drink ... and I hadn't even the courtesy to thank
 her.” -- W C Fields

“It takes only one drink to get me drunk. The trouble is, I can't
remember if it's the thirteenth, or the fourteenth.” -- George Burns

“We could certainly slow the ageing process down if it had to work its
  way through Congress.” -- Unknown

“Don't worry about avoiding temptation... As you grow older, it will
 avoid you.” -- Unknown

Doctor to patient: “I have good news and bad news. The good news is
 that you are not a hypochondriac.” -- Unknown

“By the time a man is wise enough to watch his step, he's too old to
go  anywhere.” -- Unknown