Her ass looks like two pigs fighting in a blanket.
Dumb as a box of hair.
I don’t know whether to shit or go blind. (Wha?)
He looked at me like a side-dish he didn’t order.
Cute as a bug’s ear.
Crazy as a bed bug.
Partied like a rock star.
Smoked like it was her job.
dwc1970, FL said “that boy’s about as sharp as a sack of wet mice.” I use that one a lot.
When upset about something not going her way, my mother was fond of yelling: “Well, shit and slide in it!”
“Christ on a crutch!” is one I use often as well, I can’t remember where I picked it up.
She has a face like a bag of hammers.
He/She’s bug ugly.
Shit and fall back into it.
Go piss up a rope.
Here’s a quarter, go call someone who cares.
You must have mistaken me for someone who gives a shit.
Corrections …
Drops quicker than a prom dress.
Your breath could knock a buzzard off a shit wagon.
Hotter than a pepper sprout.
Crazier than a box of fruit loops.
Quit your grinin’ and shuck your linen.
I’m so poor I can’t even pay attention.
One from my grandfather, back when I was a little zoog and would say that I wished I had one thing or another:
“Wish in one hand, and shit in the other. See which one fills up first.”
A wise old man, my grandfather.
My step-father was great for these
more full of s*** than congress at election time
flightier than a fart in a windstorm
colder 'n a witches tit in a brass bra
'bout as bright as a 10watt bulb, he is.
“Higher than a giraffe’s spotted testicles”
My boss said that last week… 
Dumber than a box of rocks.
A rock with lips.
I’ll hand him his ass in a paper sack.
Homlier than (or “as usefull as”) a sack full of assholes.
He’s so dumb he thinks the Canadian boarder pays rent!
He’s so dumb he thinks Wade Boggs is what you do before swimming across the Potomac.
Don’t just sit there like a bump on a log. (my mother’s personal favorite)
Hotter than 2 squirrels fucking in a wool sock.
Colder than a cast-iron toilet on the shady side of an iceburg.
Useless as tits on a boar.
Hotter than a 2-dollar pistol.
Slower than molasses in winter.
It’s alot like masturbating with a cheese-grater: Slightly amusing, but mostly painful.
–IDB
She’s so hot, you could fry an egg on her ass.
Or, conversely…
She’d make a steam train take a dirt road.
He knows not shit from Shinola, and his shoes show the strain.
And from the Book of Holes (Firesign Theater,)
“They knew not their holes from an ass on the ground.”
“What’s your favorite …?” threads belong in In My Humble Opinion. I’ll move this for you.
Cajun Man ~ SDMB Moderator
He/she fell off the ugly tree and hit every branch on the way down.
Rode that shit like a hobby horse.
[Said when something is not overwhelmingly great, but will do] “Well, it beats a sharp stick in the eye.”
[Said of a chesty gal] “She’s build like a brick shit house.”
All hat and no cattle…
Too poor to pay attention
Too poor to paint and too proud to whitewash.
Grinnin’ like the Baptist Preacher followin’ the rev’noor man to the crick.
(A dumb person) - The lights are on but nobody’s home.
(Just one of the greats from the movie The Big Lebowski. Asked if he wanted a drink he replies) - Does the Pope shit in the woods?
(I guess that’s only funny if you know the two standard replies) - Does a bear shit in the woods? Is the Pope Catholic?
Old enough to bleed, old enough to breed (Ugh, I don’t like that one)
Darker than a black steer’s ass on a moonless night. (Another Lebowski)
More complicated than Chinese algebra.
Young, dumb, full of cum (another bad one)
Shit like a goose
Well pee on that noise. (thanks, mom)
Christ in a sidecar!
I wouldn’t kick him out of my bed for eating crackers. (though the one time I said it, someone came back with “Why else would he be there?” courtesy of Cheers)
(Note: I’m pretty sure I heard Steve Martin ask “Does the Pope shit in the woods?” on one of his early albums, possibly Comedy is not Pretty. Still a great line.)
Welcome to the SDMB, zoog! That bit of advice would have been a worthy addition to the “Funniest thing you ever heard an old man say” thread of a few months ago.
Here are some of my favorite colloquial similes, metaphors, and other figures of speech:
Dumber than a bag of hammers
“She looks like she got hit in the face with a bag of nickels!”
A few sandwiches short of a picnic
Not the brightest bulb on the Christmas tree
As hopeless as a one-legged man in an ass-kicking contest
My dad often uses “older than baseball” or, for something/someone really ancient, “two days older than God”.
“He’s so ugly he could back a dog off a meat truck.”