Generator Land has this Homespun Analogy Generator, which will spit out funny analogies as funny as Dave Chappelle before he went nuts.
Here are some from the hall of fame:
I'm as gassy as a sick seal in a bowl of soup.
He's as grumpy as a holy badger at a pie eatin' contest.
She's as undecided as an oriental sloth with an itch.
I'm as truthful as an overweight antelope having a sneezing fit.
Link - Thanks Mike Raia!