-They screwed him tighter than a vampire’s coffin!
Fat woman in too-small clothing: “An elephant in a piss-ant’s hide.”
Ugly person: “Face looks like a forest fire that was put out with a screwdriver.” (from Terrence and Phillip)
“He’s got more money than God”
“He’s got a hair across his ass” (someone being bitchy/mad/anal retentive)
How about calling someone who talks a lot a “Chatty Cathy”? (After the original talking doll)
I always liked these:
“I see said the blind man, as he picked up his hammer and saw.” (used to denote understanding)
“He’s crazy as a pet coon.”
“He’s as useless as a footbal bat.”
“Close only counts in horseshoes and handgrenades.” (sometimes “and drive-in movies” can be tacked on the end of that one.)
“Well slap my ass and call me Sally!”
“Well pierce my ears and call me Breezy” (thanks Dad!)
“I’m outta here like Vladimir.”
“I’m out like trout.”
“Let’s do this like buddhists.”
And a personal favorite that made me laugh for about 2 hours after my Texan friend said it:
“Look at my baby cousin. Ain’t he cute as a dick.”
How about from Archie Bunker:
“Sticks and stones may break my bones, but you are one dumb pollack.” …when he was speaking to “Meat-Head”
Come on, fair suck of the sauce-bottle!
If he had any more brains, he’d be stupid!
Rubystreak:
“Brick shithouse” (in Oz) was in the backyard/is out in a paddock, and ever so slightly over-engineered for longevity.
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Its hotter than a freshly f*&%ed fox in a forest fire.
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Hotter than a blistered pussy in a pepper patch.
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He’d worry the horns off of a billy goat.
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Grinnin like a possum eatin saw briars.
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Ugly enough to make a bulldog run up a plate glass wall.
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Dumber than a box of rocks.
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Nastier than two boys screwing in church.
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Sweatin like a whore on the front pew.
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Now there’s a turd in the punchbowl.
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He’d squeeze a nickel till the Buffalo shits in his hand.
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Meaner than a rattlesnake with a toothache.
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Like running through hell wearing gasoline underwear.
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I’ll put my foot in your ass up to the adenoids.
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That boy is slower than smoke off shit.
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Stomp a mudhole in your ass and walk it dry.
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I’ll jump on you so quick you’ll holler, “Ya’ll quit”, and there won’t be nobody but me.
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She as pretty as a speckled pup.
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She’s finer than frog hair split three ways.
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She could suck the chrome off a trailer hitch.
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I’ll give you a Yankee dime. (meaning a kiss.)
I’ve got a million of em.
“Fell out of the ugly tree and hit all the branches on the way down” is one of my perennial favorites.
I’m also quite fond of this extension of “like white on rice”: “like white on rice on a paper plate with a glass of milk and a polar bear in a snowstorm”. Needless to say, I find hyperbole amusing. 
On Futility:
“Like throwing bricks in the Grand Canyon”
On Intelligence:
“Sharp as a bag of wet mice”
Nervous as a cat with deaf kittens.
In response to “F*ck me running!” – “Take off!”
I’ve always heard that one this way: “When they were handing out brains, you thought they said trains and asked for a slow one.”
That’s been around since God was a boy.
That was hilarious!
My favorite is thanks to Goo
Here’s some of mine
“That boy’s as sharp as a marble”
“Cry me a river, build me a bridge and GET OVER IT!”
“When your IQ gets to 50, SELL!”
“His IQ hit rock bottom and he started to dig”
“Were you potty trained at gunpoint?”
“Somebody pissed in his gene pool”
" I couldn’t fancy him if his arse was studded with diamonds!"
I’m built like one o’ those. It’s not generally a complement. Think solid, and not at all curvy.
How 'bout:
(someone putting on airs) There’s mutton dressed as lamb.
(being overly cautious or overdoing a task) That’s like sewing your suspenders to your belt.
On astonishment:
Well, Christ all Friday/Christ in a Waffle House!
On size:
He’s hung like a field mouse/wine cork/baby carrot
On futility:
That’s like an ant crawling up an elephant’s leg with intentions of rape.
On looking for something/someone:
I’ve been all over hell and half of Georgia.
A threat:
I’ll kill ya and tell God you died.
On intelligence:
He’s got all the brains God gave a head of lettuce.
On excitability:
He’s got a panic button on his forehead that goes off every time he bumps into a wall.
On sex:
Make you walk across the floor like a water spider.
Making the sign of the two-headed aardvark.
(The credit for this one goes to Joe-Bob Briggs.)
Ugly Person
Got a face that would make a train take a dirt road.
I’m sweatin’ like a whore in church.
Don’t piss on my shoes and tell me it’s rainin’.
(On going to poop) “Gonna drop the kids off at the pool”
(On road rage) “You drive like old people f*ck…slow and sloppy!”
I’ll be dipped. Someone remembered that. It’s worse than you remembered, though, because it was from the other point of view. When I tried to help with his part of a project, he said, “I’m f***ing this chicken–you’re just holding the wings.”
Yes, it’s extremely crude…and very funny. 
Others:
Of an old person/thing:
“Ninety years older than God.”
Of a small room/area:
“So small you can’t cuss a cat without getting fur in your mouth.”
“Too small to swing a cat.”
One I haven’t seen yet
Noiser than two skeletons f***ing on a tin roof.
Three very old ones:
They sure ruined a good asshole when they gave you teeth.
He was so scared they couldn’t pull a needle out of his ass with a tractor.
As nervous as a pregnant nun.