I discovered this a long time ago, but I still think it's worth posting about:
I used to be incredibly pessimistic, cynical, and sarcastic, and then I discovered XKCD. XKCD is also sarcastic and a little cynical sometimes, but the character Beret Guy is always naively optimistic (and endowed with super powers such that he always wins arguments against other characters even when wrong, but that's a different issue).
After reading XKCD for a little while, I realized "Hey, being pessimistic makes me sad. I'm going to take Beret Guy's view of life." It was a little less sudden than that, but not by much. I'm now an optimistic individual, even if I'm still ridiculously sarastic sometimes.
Here's the comic which best illustrates that optimism.