From some of my favorite films:

"Some things just get meaner as they get older."
"First rule of Italian driving...What's behind me is not important."
"No Mr. Bond, I expect you to die."

Well, can I quote my dad:

"Son, the one thing you must understand... is that you will never understand women"

My wife would probably concur...

My grandfather once said:

"All it cost me to get married was $5 for the marriage license and all the money I made for the rest of my life."

My favorite line was to me from Ballard Haiden, an elderly gun collector in Virginia who sold me my first pistol many (too many) years ago.
When I got home, in the box was a note in thick pencil, on grade-school paper:

..."be reel keerful with this here gun, and don't never shoot nobody without they need it.."

By 1791, Alessandro Volta realized that the dead frog could be replaced by cardboard soaked in salt water.
From "The History of Electricity"

Whenever I get frustrated because something isn't working this line always pops into my head.

-Blast

A couple of other good ones from films:

"You keep using that word. I do not think it means what you think it means." (The Princess Bride)

"Why did you come to Casablanca, Rick?"
"I came for the waters."
"There are no waters in Casablanca."
"I was misinformed."
(Casablanca)

"He'll regret it to his dying day, if ever he lives that long." (The Quiet Man)

1) "Life is not a journey to the grave with the intention of arriving safely in a pretty and well-preserved body, but rather to skid in sideways, thoroughly used up, totally worn out, and loudly proclaiming "wow - what a ride!"

2) "It is what it is." Weather is not good or bad - it is what it is - we make it good or bad. Your day is not good or bad...

"Whiskey is man's best friend. Whiskey is bottled dog." -Vinicius de Moraes

I don't really know where I head or picked it up from but it is somthing I have to rremind some people of from time to time.
"Don't mistake my kindness as weakness"

Mongol Leader: "What is best in life?" Conan: "To crush your enemies, see them driven before you, and hear the lamentations of their women."

Unknown: "A hundred years from now, it will not matter how large my bank account was, what sort of house I lived in, what kind of car I drove, but the world may be different because I was important in the life of a child."

Travis Dane: "Assumption is the mother of all screw-ups" (edited)

Teddy Roosevelt: "There is no room in this country for hyphenated Americanism. When I refer to hyphenated Americans, I do not refer to naturalized Americans. Some of the very best Americans I have ever known were naturalized Americans, Americans born abroad. But a hyphenated American is not an American at all.
This is just as true of the man who puts “native” before the hyphen as of the man who puts German or Irish or English or French before the hyphen. Americanism is a matter of the spirit and of the soul. Our allegiance must be purely to the United States. We must unsparingly condemn any man who holds any other allegiance.
But if he is heartily and singly loyal to this Republic, then no matter where he was born, he is just as good an American as any one else.
The one absolutely certain way of bringing this nation to ruin, of preventing all possibility of its continuing to be a nation at all, would be to permit it to become a tangle of squabbling nationalities, an intricate knot of German-Americans, Irish-Americans, English- Americans, French-Americans, Scandinavian- Americans, or Italian-Americans, each preserving its separate nationality, each at heart feeling more sympathy with Europeans of that nationality than with the other citizens of the American Republic.
The men who do not become Americans and nothing else are hyphenated Americans; and there ought to be no room for them in this country. The man who calls himself an American citizen and who yet shows by his actions that he is primarily the citizen of a foreign land, plays a thoroughly mischievous part in the life of our body politic. He has no place here; and the sooner he returns to the land to which he feels his real heart-allegiance, the better it will be for every good American."