I’d pick my family’s motto:
What’s your back-up plan?
Or something my son, the computer geek, sadly said long ago:
Life would be easier if you could hack it.
Not that I’d ever be caught dead in a t-shirt. Maybe a nice silk charmeuse, close fitting blouse—with long sleeves and a jewel neckline—in a cream color, or maybe a daring aubergine—and a pair of really, really great heels—a long wool crepe skirt—and a bag to match…fabulous earrings, demure bracelet…
Oh, I’m sorry. I must have been dreaming about how I’d sum up my FASHION philosophy. My bad. Carry on.
Man these are great
Not comical but…
“God will understand, and if he does not, then we have nothing to worry about, since he is not God” - Kingdom of Heaven
I like the “I regret Nothing” one
how about:
Front: I don’t care if I die
Back: After all, I’ll be dead, so how could I?
FIDO
F*ck it, drive on.
Thank you! And have a watch is just as awesomely up there as having a barcode, ‘stitches’ or showing off your bionic biceps. I LOVE it!! Anyway, great minds and all that, right? 
Ok, maybe just yours then… 
Before you get it tattooed onto anything, you might want to change that to “dum vivimus, vivamus.” As it stands, it’s something of a tautology: While we live, we are alive. I’m sure there are some circumstances when this might be appropriate.
In a similar vein is Catullus’ injunction to Lesbia, vivamus atque amemus (let us live, and let us love), which seems like as good a motto as any. And as Lissa can attest, it makes you look really smart.
My serious answer is :
“I don’t care who you screw or who you worship, just leave me alone.”
My humorous answer is :
Joe
Some possibilities:
RTFM (stands for Read The Fucking Manual, a commonly heard phrase in tech support break rooms)
Don’t believe anything you read on a t-shirt
Darwin speaks to me
(front) Jesus Saves
(back) … and takes half damage (picture of a d20)
There’s only one other person in the world who’d get the real meaning to this phrase, and she’s not a Doper. But – it’s what I’d like to put on a T-shirt some day an’ wear it: “Ain’t life a jazz?”
To me, life certainly is. 
I spend most of my day in cubicle-based beauracracy, so mine would be:
“Power Corrupts; PowerPoint Corrupts Absolutely”
You might be interested in this drawing, which I recently DID pick up on a T-shirt:
Ceci n’est pas une drille
http://content.perspicuity.com/?q=node/330
Another that I like (but it is not on me) is a hand grenade a guy has on his chest. The “pin” of the grenade is the CBR in his nipple.
Dyslexics of the world
UNTIE
10% of all proceeds from this shirt go to benefit the Sunny von Bulow Chair at Schrodinger University.
My new T-shirt philosophical summation:
I’M STILL THINKING ABOUT IT.
“People say that what we’re all seeking is a meaning for life. I think that what we’re seeking is an experience of being alive, so that our life experiences on the purely physical plane will have resonances within our own innermost being and reality, so that we actually feel the rapture of being alive.”
- Joseph Campbell
Now that’s just excellent. And I’d ask if you’d seen any other ‘interesting’ tattoos, but I figure I’d better not keep on hijacking the thread. Back to t-shirt philosophy everyone!
To Love Another Person Is To See The Face Of God
–Les Miz
Ooh, me like.
“Where you come from is gone. Where you thought you were going to weren’t never there. And where you are ain’t no good unless you can get away from it.”
Play like a dog.
Laugh like a kid.
Sleep like a cat.