Put up or shut up.
Why do today what you can put off till tomorrow? That saw me through my entire academic career.
This is a book I’ve never read but whose title I admire: Feel the fear and do it anyway – a great motto for times of high anxiety.
From the Notebooks of Lazarus Long: You have taken yourself too seriously.
And my own: Live without fear, die without regrets. (and haunt everyone who pissed you off)
Yes, that’s a good one. But I prefer to state it “Contrition is easier than persmission.” Rhymes, you see. 
A little bit sappy, but…
“To Love Another Person is to See the Face of God”
“A fool can ask more questions in a minute than 12 wise men can answer in an hour.”
“Don’t piss down my back and tell me it’s raining.”
It’s in my signature.
“Only two things are infinite: the universe, and human stupidity. And I’m not sure about the universe.” Helps me feel much better about myself when I’m sitting there working on something and getting exactly nowhere.
Or, in the words of the immortal Red Green, “If it ain’t broke, you’re not trying.”
There are a lot of the original demotivators (the ones on despair.com) near my cube. And not that many inspirational sayings around, either…
My motto is “Excellence is not an act, but a habit.” This is something very close to something else that Aristotle once said.
“Be who you are and say what you feel because those who mind don’t matter, and those who matter don’t mind.”
I think that’s from Dr. Seuss.
I frequently use these quotes to maintain some semblance of harmony in my life:
“Peace comes from within. Do not seek it without.”
-Buddha
“Worry is an insufficient cycle of thought revolving about a pivot of fear.”
-Unknown
Only two good ones…
“God hates a coward” and to follow that up, “It’s not a mistake, it’s a learning experience”.
My personal favorite:
If it’s stupid but it works, it’s not stupid.
I have a few:
“You never know until you try.”
“A pint can’t hold a quart. If it’s holding a pint, it’s doing the best it can.”
“If it doesn’t cause me pain, make me bleed or cost me money… screw it.”
“95 percent of everything is crap.”
“Go toward the fear.” I kind of like that one, from someone who argued that if anything frightens you, you should do/confront it immediately.
“It’s never too late to have a happy childhood.”
“Chance favors the prepared mind.” (Louis Pasteur)
“Lead, follow, or get out of the way.” (seen on bumper sticker)
My motto is the same as my blood type: B-positive. It doesn’t always work, but I try.
“Anything worth doing is worth overdoing”
“The only wisdom we can hope to acquire/Is the wisdom of humility: humility is endless”
“The only way to get up every morning with a smile on your face is to go to bed with a coat hanger in your mouth.”