Uglier than (or plainer than) a board fence.
I didn’t know whether to shit or go blind, so I closed one eye and farted.
I was so scared I was shittin’ rubber nickles.
I was so mad I coulda spit brass tacks.
I gotta pee so bad my back teeth are floatin’.
You drink any more coffee, and we’re gonna’ have to row home.
“Axe handles” are a common unit of measurement, ie: My ass is headin’ for two axe handles wide.
A southern pal would say “it’s hotter than a well-digger’s crotch on the 4th of July!”
Another southern co-worker from way back commented on a lazy colleague: “He’s about as useful as tits on a boar-hog!”
From my dad: “I’ll be all over you like a cold sweat” (A vague threat.)
Referring to a wide butt: “Two axe handles and a long squirrel jump.”
“Dumber than a box of hair” has an odd, prosaic quality that’s always appealed to me.
Busier than a cross-eyed cranberry picker.
Dumber than a box of rocks.
Slicker than snot on a doorknob.
I’m gonna be on you like white on rice.
Dryer than cracker juice!
Richard Nixon once said during a David Frost interview that something “was as thin as piss on a rock” referring to something insubstantial. If only his tapes had that kind of profanity on them, they’d be a heck of a lot more interesting.
“How’re you?”
“Finer than a frog hair split four ways.”
I’ve known a few catfish, all mouth and no brain.
My mother modeled herself after Emily Post. All the same when a younger mom in the neighborhood once asked her if she should address her by her first or last name, Mom said “Honey, you can call me a son of a bitch just so long as you smile!” In Mom’s defense, this was decades ago.
HEY! I resent that remark. 
-Dirty
More have occurred to me whilst reading this thread:
My dad used to say, “That store is closed up tighter than a bull’s ass in fly season,” or “She’s uglier than a mud fence full of water dogs.”
A former co-worker would say, “You could f**k up a two-car fiuneral.”
I always loved the alliterative quality of “hotter than a half-f**ked fox in a forest fire.”
Carry on.
I’m suprised nobody mentioned my favorite- Faster than greased lightning. As far as I’m concerned, there is nothing faster than greased lightning (I mean, come on )
I also heard “That’s as hard as a one legged cat trying to take a dump on a frozen lake” don’t remember where it came from tho.
S/he’s dumber than a hoe handle.
(Referring to a stupid act or an ugly person) Now I know why tigers eat their young.
I heard it as the even more alliterative “hotter than a freshly-fucked fox in a forest fire.”
It could also be “hotter than a six-peckered billy goat in a pepper patch.”
Dr. J
Here’s a few of my fav’s in another thread
He’s as sharp as a mud fence.
Courtesy of Stephen King:
“You need that like a hen needs a flag.”
“He’s as gay as your dad’s old hatband.”
Referring to someone with buck teeth - “He/She could eat watermelon thru a picket fence.”
My dad has said the following:
“Slower than molasses in January.”
On something good happening:
“That was better than a poke in the eye with a sharp stick.”
And my personal favorite:
“Crazy as 'coon shit.” 