Here’s mine:
“The life of every man is a diary in which he means to write one story, but writes another; and his humblest hour is when he compares the volume as it is with what he vowed to make it.”
J.M. Barrie, The Little Minister
“If you can keep your expectations tiny, you’ll go through life without being so whiny.”
M. Groening (from one of his … is Hell books)
I had that posted in my office and I swear that was the only way I got through the department’s every-3-months total reorganization.
“It is better to be silent and though a fool than to open your mouth and remove all doubt.”
If everyone lived by this, we’d all be better off.
“if in doubt, put it out!”
Tony Adams, 1991
A maxim I think we all live by!
“it is better to make a bad decision than no decision at all”
Voltaire?
That’s my pompous one.
“U can’t touch this”
MC Hammer
Sage words, I think we all agree.
To select just one favorite is, well, it’s impossible I say. But here’s one that I like a lot:
“If I were to wish for anything, I should not wish for wealth or power, but for the passionate sense of the potential, for the ever young and ardent which sees the possible. Pleasure disappoints, potential never. And what wine is sparkling, what so fragrant, what so intoxicating, as possibility?!”
–Soren Kierkegaard
Mile by mile, life’s a trial.
Yard by yard, life is hard.
Inch by inch, life’s a cinch.
Ok, it’s corny. But it helps me keep things in perspective.
One of my favorites:
“If we believe absurdities, we shall commit atrocities.” --Voltaire
I don’t know who said this, but it has always been one of my favorites.
“Why not go out on a limb, isn’t that where the fruit is”?
Where there is no bison, the people perish.
When I was a teenager I spent several weeks at a Christian mission in NW London, and above the door of the chapel they had this quote from the book of Proverbs in rather ornate calligraphy. The actual word is “vision,” not “bison”, but the way the letters were written made the v look like a b. So that’s the way we quoted it.
My wife says it’s prophetic of the fate of the Native Americans.
Oh, everything’s too damned expensive these days. This Bible cost 15 bucks! And talk about a preachy book! Everybody’s a sinner! Except this guy.
–Homer Simpson
“If it doesn’t fit, you must acquit.”
God bless poetry like that. A tear, I tell ya’. It brings a tear to my eye. 
Must be Voltaire day at the SDMB. Here’s a third one from him:
“Love truth, but pardon error.”
“I cannot be making a mistake. —But some day, rightly or wrongly, I may think I realize I was not competent to judge.” ~~Wittgenstein, On Certainty
It is not my favorite because of its universal applicability (which is non-existent), but rather because of another comment he makes in the book: when we talk about what we know, we often forget the expression, “I thought I knew…” 
Beer is proof that God loves us and wants us to be happy
- Benjamin Franklin
How can I miss you if you won’t go away?
Oh, I can come up with a few of them:
“When in trouble or in doubt, run in circles, scream and shout”
- RAH (me thinks)
“There are no good or evil people. In fact, we’re all evil. Some just happens to be on our side.”
(Maybe Terry Pratchett, I found it there)
“Men talk to women to get laid. Women have sex with men to get someone to talk with.”
(maybe misquoth from J. McInerney)
What? Me? Cynic? Nah…
My entry in the Voltaire olympics:
“God is a comedian with an audience too afraid to laugh”
My quotes to guide my daily life:
“Whateva, I do what I want” - Eric Cartman
“We wanna have fun, and we wanna get wasted” - andrew wk
“The Dude abides” - Jeffrey Lebowski
“When a diplomat says yes, he means maybe; when he says maybe, he means no; when he says no, he is no longer a diplomat. When a lady says no, she means maybe; when she says maybe, she means yes; when she says yes, she is no longer a lady.” - from Forbes magazine, I’ll dig up the old issue to figure out who said it (it’s from the 18th C. IIRC).
“We are all spiritual beings with temporary physical shells that too often seem to define our actions and emotions.” - Serj Tankian of SOAD
I sort of collect quotes… when I find something I like in a book or somewhere, I write it down. My favorite quote of all time is this one, because it decribes me very well (I think):
I use the last sentence of that one quite often.
Oh, and the one in my sig is a current favorite.
“Don’t mistake the top of your rut for the horizon.” --James Patterson
“It’s always darkest just before the piano falls on you.” --Oliver Faltz (one of my pseudonyms)
“If you know what you’re doing, it’s not an adventure.” – Me, actually.
“It’s okay to have low self-esteem if everything you do is wrong.” – Me, again.