I have this one taped to my wall.

"The problem with the Internet Startup craze isn't that too many people are starting companies; it's that too many people aren't sticking with it. That's somewhat understandable, because there are many moments that are filled with despair and agony, when you have to fire people and cancel things and deal with very difficult situations. That's when you find out who you are and what your values are." --Steve Jobs

When things get tough, I read that and remind myself to keep going.