This bumpersticker I would say fits me pretty well: "I'm not suffering from insanity, I'm enjoying every minute of it."
"You are what you eat, don't be a vegetable."
And Cae, although that quote from Independence Day is pretty funny, I think these three are better (it should be obvious what they're from):
Monk: And the Lord spoke, saying, "First shalt thou take out the Holy Pin. Then, shalt thou count to three, no more, no less. Three shalt be the number thou shalt count, and the number of the counting shalt be three. Four shalt thou not count, neither count thou two, excepting that thou then proceed to three. Five is right out! Once the number three, being the third number be reached, then lobbest thou thy Holy Hand Grenade of Antioch towards thou foe, who, being naughty in my sight, shall snuff it."
and
King Arthur: Now stand aside, worthy adversary.
The Black Knight: 'Tis but a scratch.
King Arthur: A scratch?! Your arm's off.
The Black Knight: No it isn't.
King Arthur: Then what's that then.
The Black Knight: I've had worse.
and
The Black Knight: Right, I'll do you for that.
King Arthur: You'll what?
The Black Knight: Come here.
King Arthur: What are you gonna do, bleed on me?
The Black Knight: I'm invincible!
King Arthur: You're a loony.
and then the website
www.despair.com has a lot of funny demotivators.