Speaking of Scott McCloud, check out this great tool found on his blog.  The Grimace Project is based on an idea he talks about in his book Making Comics - that various complex emotions can be represented through a combination of six basic emotional states including; joy, surprise, fear, sadness, disgust, and anger.  This fun little tool allows you to explore various facial expressions to get the right emotional state for the character or person in your artwork.  The above image illustrates a combination of joy and disgust, which is what I expect many of you experience when you read the posts here.   Anyway - I suppose a mirror would work as well, but how often do you get to play with somebody else's face?