Here comes a few I've seen in print or heard:
Busy as a one-armed paperhanger with crabs. - exceedingly busy person.
Cold enough to freeze the brass of a bald monkey. - very cold.
He doesn't know whether to scratch his watch or wind his ass. - indecisive person.
Don't tell more than six. - don't tell anybody.
Don't wear it - eat it. - this to a sloppy eater.
I'm so hungry I could eat a horse and wagon and then chase the driver.
All over the place like a madwomans shit. - an untidy person/place.
He would talk a glass-eye to sleep. - a bore.
You can hear them change their minds. - thin walls.