Fire bad; tree pretty" to "Well, you were myth-taken," there is--as Billie has demonstrated--a Buffy quote for every occasion. But Buffy is not the only show brimming with useful, quotable lines or decent life lessons.
For instance, I base my own philosophy on "Life is pain, highness. Anyone who says differently is selling something." But that's just me.
What about you? Is there a pithy quote from a TV show, film, or even (gasp!) work of literature that you find yourself using often, or completely and wholeheartedly agreeing with? Buffy quotes are welcome, but so are quotes from anything else!