One of my favorites has always been "he/she/it is so ugly, he/she/it would make a freight train take a dirt road." Another is "a carpenters dream, flat as a pancake, and never been nailed." (Refers to a very young, innocent woman)
When some one was lying, down south, I always heard "that dog don't hunt". For someone who worries about everything that might happen..."when you hear hoof beats, think horses, not zebras".