A compilation of some really great graffiti:

If you can piss this high, join the fire department.
  -On the wall in the men's restroom at a height of 6 feet.  O'Ryan's 
Irish Pub.  Ashland, Oregon.

Beauty is only a light switch away.
  -Perkins Library.  Duke University.  Durham, North Carolina. 

I've decided that to raise my grades I must lower my standards.
  -Houghton Library, Harvard University.  Cambridge, Massachusetts. 

If life is a waste of time, and time is a waste of life, then let's 
 all  get wasted  together and have the time of our lives.
  -Armand's Pizza.  Washington, D.C. 

If Bush were captain of the Titanic, he'd say we were stopping for ice. 
  -Smoky Joe's.  Philadelphia, Pennsylvania.

Remember, it's not, "How high are you?" it's "Hi, how are you?" 
  -Rest stop off Route 81.  West Virginia.

God made pot.  Man made beer.  Who do you trust? 
  -The Irish Times.  Washington, D.C.

Fighting for peace is like screwing for virginity. 
  -The Bayou, Baton Rouge, Louisiana.

No matter how good she looks, some other guy is sick and tired of 
putting up with her shit.
  -Men's Room, Linda's Bar and Grill.  Chapel Hill,  North Carolina. 

To do is to be.       -Descartes
To be is to do.       -Voltaire
Do be do be do.     -Frank Sinatra
  -Men's restroom, Greasewood Flats.  Scottsdale,  Arizona. 

At the feast of ego, everyone leaves hungry.
  -Bentley's House of Coffee and Tea, Tucson,  Arizona. 

It's hard to make a comeback when you haven't been anywhere.
  -Written in the dust on the back of a bus. Wickenburg, Arizona. 

Make love, not war.  --Hell, do both, get married!
  -Women's restroom, The Filling Station.  Bozeman, Montana. 

God is dead.  -Nietzsche
Nietzsche is dead.  -God
  -The Tombs Restaurant.  Washington, D.C. 

If voting could really change things, it would be illegal. 
  -Revolution Books.  New York, New York.

A Woman's Rule of Thumb:  If it has tires or testicles, you're going 
 to have trouble with it.
  -Women's restroom, Dick's Last Resort.  Dallas, Texas. 

JESUS SAVES!
But wouldn't it be better if he had invested?
  -Men's restroom, American University.  Washington, D.C. 

If pro is opposite of con, then what is the opposite of progress? 
Congress!
  -Men's restroom, House of Representatives.  Washington, D.C. 

What are you looking up on the wall for?  The joke is in your hands. 
  -Men's restroom, Lynagh's.  Lexington, Kentucky.