Here are the two I use the most:
"If you can't say something nice, don't say anything at all." Thumper's mom in Bambi. I don't know how many times I've told my kids that in so many situations this is really good advice. I've definitely followed this advice myself many times.
And I hate to admit it, but my kids get a smile on their face and get it when I say, "Life is full of tough choices." Ursula from Little Mermaid. This seems to come out of my mouth when they don't like their options, but really and truly do know what they need to do.