from the Hitchhiker’s series
Don’t Panic!
Never drink more than two Pan Galactic Gargle Blasters unless you are a thirty-ton mega-elephant with bronchial pneumonia
Anyone capable of being made President should be on no account allowed to do the job
the purpose of the President is not to wield power, but to draw attention away from it
Please do not press this button again!
Good ones!
From A Song of Ice and Fire, book three I think:
“A man can own a woman or he can own a knife, but not both”. Said by a wildling girl, who’d know.
Jane Austen was only being partially ironic when she wrote:
"She was heartily ashamed of her ignorance. A misplaced shame. Where people wish to attach, they should always be ignorant. To come with a well-informed mind is to come with an inability of administering to the vanity of others, which a sensible person would always wish to avoid. A woman especially, if she have the misfortune of knowing anything, should conceal it as well as she can.
The advantages of natural folly in a beautiful girl have been already set forth by the capital pen of a sister author; – and to her treatment of the subject I will only add, in justice to men, that though to the larger and more trifling part of the sex, imbecility in females is a great enhancement of their personal charms, there is a portion of them too reasonable and too well informed themselves to desire anything more in woman than ignorance."
I wish I had known this when I was younger. I was always too much of a know-it-all and if I had pretended to be a dummy I’m sure guys would have liked me more.
But they would be the sort of men that you are better staying away from. A man who’s looking for dummy is either looking for a quick screw he can walk away from afterward, or is an abuser looking for someone who can’t fight back. Attracting a man by playing dumb IMHO is like trying to attract a pet by walking into a forest with raw meat tied around your neck. Whatever you find probably won’t be all cuddly and friendly.
“There ain’t no such thing as a free lunch” are wise words from a Robert A. Heinlein novel. Though I’ve never forgotten the advice, I have, alas, forgotten which novel it appears in - The Moon is a Harsh Mistress, maybe? TANSTAAFL, can you clarify?
“Cease to lament for that thou canst not help, and study help for that which thou lament’st.” (Willy Shakespeare, Two Gentlemen of Verona.)
Or, if you can fix it, do it; if you can’t fix it, stop fretting about it. There’s been many a time I’ve had to repeat that to myself to calm down and/or stop chewing my nails to the quick. It helps that it scans. 
“Never start a land war in Asia.” I’ve lived my entire life by that motto.
More Heinlein from Starship Troopers*: “Never pass up a good thing.”
The hard part is telling a good thing apart from what looks like a good thing but that will come back to bite you in the ass.
*I know it was in the movie; can’t remember if it was in the book.
So your advice is to pretend to be someone you’re not so boys would like you? :dubious:
I am considered smart. I took AP courses in school and consistenly made honor roll. People will not play Trivial Pursuit with me more than once. I can hold my end in a debate. And I stood out in a crowd so Ivylad, used to blonde bimbos with big chests who would sleep with him on the first date, was intrigued.
We’ve been married 17 years.
Trust me, be smart. The good guys are the ones who will appreciate your intelligence. The others can go hang.
Aaarggh! Bitten in the ass by Guadere’s Law!
[del]consistenly[/del]
consistently
Robert Heinlein, in Time Enough For Love, included several interludes containing excerpts from the notebooks of Lazarus Long. Many of them are noteworthy and wonderful advice, including:
[ul]
[li]Always store beer in a dark place.[/li][li]Rub her feet.[/li][li]If it can’t be expressed in figures, it is not science; it is opinion.[/li][li]It is better to copulate than never. ;)[/li][li]Everything in excess! To enjoy the flavor of life, take big bites. Moderation is for monks.[/li][li]Moving parts in rubbing contact require lubrication to avoid excessive wear. Honorifics and formal politeness provide lubrication where people rub together. Often the very young, the untraveled, the naive, the unsophisticated deplore these formalities as “empty,” “meaningless,” or “dishonest,” and scorn to use them. No matter how “pure” their motives, they thereby throw sand into machinery that does not work too well at best.[/li][/ul]
The second on that list is advice that can take you through many difficult situations in life.
I’m not sure if this one counts as “advice” under the OPs definition, but it’s a great quote from the same source:
[ul]
[li]Anyone who cannot cope with mathematics is not fully human. At best he is a tolerable subhuman who has learned to wear shoes, bathe, and not make messes in the house.[/li][/ul]
Larry Niven likes to dish out philosophical life-guidance advice and I’m ashamed to say that I have been a sucker for some of it. I suspect some of it is not even original to him, but that’s where I got it; two items that spring to mind are:
“You have to do this, so why not now?” (which I find practically useful for stopping procrastination)
(paraphrasing)“Never waste calories; if you must get fat, don’t achieve it using inferior food”
Don’t! Date! Robots!
(From Futurama.)
/me snertles at the multi-honor-roll student bragging about her intelligence. 
"The first sign of a nervous breakdown is when you start thinking your work is terribly important. "—Milo Bloom
One of the few gems of fiction quotes that I’ve collected that qualifies as A) “Advice,” B) “Under a paragraph long,” and C) “Doesn’t make Ran sound like a psychopath.”
“Pain, or damage, don’t end the world. Or despair. Or fuckin’ beatings. The world ends when you’re dead. Until then, you’ve got a lot more punishment in store. Stand it like a man, and give some back.”
One of life’s harder lessons.
Yes, I was a nerd. I’m proud of it. I sat in the front of the class, did my homework on Friday nights, and trooped down to the guidance office (as did other nerds) to get our class rankings once the report cards were posted.
Be kind to nerds. You will end up working for one some day. 
I like this one.
Never go in against a Sicilian when death is on the line!
“Beware of enterprises that require new clothing” - Henry David Thoreau.
Oops, just realized the OP calls for advice given in fiction. HDT quote above doesn’t count. Please disregard.