Do what’s right, do no harm, and keep the faith that by doing so, in some small measure, you will make the world a better place.
Ad astra per aspera
Or - the version sized to fit on a T-shirt:
(With the dubious smilie. Gotta have the dubious smilie.)
The Zen philosopher Basho wrote A flute without holes, is not a flute. A donut without a hole, is a Danish.
Truely words to live by.
I take mine from Red Green: Quondo Omni Flunkus Mortati
some do it naturally
some obscenely
everywhere.
(On edit: also “Stagger onward, rejoicing.” -W.H. Auden)
And so it goes.
(With a tip of the hat to both Kurt Vonnegut and Linda Ellerbee)
Personal motto: Flectere si nequeo superos, Acheronta movebo. (with thanks to Virgil)
“If I cannot move Heaven I’ll raise Hell”
Classroom motto: Disce Aut Morere.
“Learn or die.”
Mine used to be ‘How hard can it be?’
But I’m thinking of changing it to “what the fuck is this now?”
That’s pretty much mine too, sometimes followed by “Any day above ground is a good one.”
People worry too much over things they can’t control.
Don’t sweat the petty things and don’t pet the sweaty things.
“If you run, you’ll just die tired” meaning: bullets are supersonic so if you’ve heard a shot it’s already missed or life is a train and you’re in a tunnel and/or plan ahead because by the time it happens it’s too late to prepare.
Or
Vide Audi Tace
No worries.
This isn’t the end of the adventure. It’s the beginning.
“I will not rest until the Earth has been conquered, I have been acclaimed God-King, Elizabeth Mitchell and Kate Winslett have given birth to my heirs simultaneously (to ensure a civil war after my death), and everyone involved with the latter four seasons of Star Trek: Voyager has been executed. Except for Roxanne Biggs-Dawson, who is spared only to bear the spoiler heir.”
Smile to the world, and the world usually smiles back at you 
You only live once.
I work in the transplant department and see a lot of sick people who thought they were going to live a whole lot longer.
“Life is lumpy. But a lump in your oatmeal, a lump in your throat and a lump in your breast are not the same kinds of lumps. Learn to recognize the difference and act accordingly.”
When you take the time to stop and analyze it, most problems are really just lumps in your oatmeal.
I have no idea if the Latin is correct, but my motto is:
*Malam cerevisiam faeceus in cathedram stercoris *
Which is supposed to mean “Let he who makes bad beer be thrown on the town dung heap”
(I am the quality assurance lab guy at a brewery, and an avid homebrewer)
"When life gives you lemons, just shut up and eat your goddam lemons!!"