Well, call me zombie and shovel-slap me silly!
This one ended a business meeting I attended. An earnest young man in a grey suit had made his presentation, which incidentally claimed credit for a number of things which had been initiated by other people. The bossman leaned back in his chair, put his hands behind his head, stared at the ceiling and said:
"Son, that puts me in mind of a flea
floating down the chattahoochee on a oak leaf
masturbatin’ an’ yellin’, RAISE THE DRAWBRIDGE!!!
If you ever come in here again claiming credit for other people’s work I will personally escort you to the door. Are we clear?"
We all left quietly.
He couldn’t get laid in a monkey whorehouse with a sack of bananas.
It’s hot as a crotch
I’m full as a tick
Better than a poke in the eye with a sharp stick
He licks the window on the short bus
Whatever creams your twinkie
Christ on a crutch
Family and Regional (Southern) favorites: "You’re moving like dead flies are falling off of you"
(Implication: you’re moving too damn slow. I guess if you moved faster either the dead flies would stay put or live flies would fall off of you instead?)
"…then you cut the English Muffin half in two and toast it"
(Good thing you specified, I was gonna cut it half in THREE)
"Go hug your Aunt Lucille’s neck"
(and the rest of her too, I reckon… don’t strangle her or anything either, ok?)
This is not really a colloquialism, but a phrase invented by my Spanish-speaking preschool client that I am determined to enter into the Spanish idiom.
“Echar el taco” (roughly meaning: to expel the taco) = vomit.
Originated when said preschooler’s little sister had eaten too much Taco Bell and the inevitable occurred.
Proposed usage: “Juan wasn’t feeling well on the ride home. Echó el taco all over the back seat.”
He’s as useless as a screen door on a submarine.
She musta ate Dumb Flakes for breakfast.
Use your head for something besides hangin’ your hat.
She’s a few cookies short of a Deluxe Assortment.
My dad used to say, “They looked at me like I was the bastard at the family reunion.”
My personal favorite 
“She’s so ulgy, it looks like her face caught on fire and someone tried to beat it out with a wet chain.”
“I’m busier than a one-armed pimp in a bitch slappin’ contest.”
Everyone knows that everybody pisses in the pool, but just try it from the divingboard and see what happens.
In English this could be ‘tossed the taco’!
Inconspicuous as a dog turd in a bowl of milk…
So hungry you could eat a horse and chase the rider…
instead of “do bears shit in the woods?” Does Angelina Jolie have lips?
Steak too rare?- I’ve seen cows hurt this bad get well… Not rare enough?- Cut his horns off, wipe his ass, and bring him out…
That went over like a fart in a spacesuit…
When a woman doesn’t want to do something “I’d rather have a double mastectomy”…
When pigs fly and carry passengers…
I love these. 
“Cry-baby, titty-mouse!” (as in you are one/I]
“Hungry as a peach orchard boar” (also works for “Wild as…”)
"Fck me a’runnin’!"
“As good as you are and as bad as I am… I’m as good as you are as bad as I am!”
“Santie Claus”
“Colder than a well-diggers ass!”
“As unaccustomed as I am to social drinking, perhaps I will have just one.”
“Shit-fuzzy!”
“You’re a good hand”
I’d rather have a mobile desk next to me than a double mastectomy.
I’d been waiting for the right opportunity to say that, but despite what you may have heard, opportunity often does not knock at all.
As cold and dead as a six-year-old thread.
A couple of favorite insults from a politician I used to work for:
*You, sir, are the north end of a horse going south.
You’re lower than a snake’s tail in a wagon rut.*
He used to sprinkle these liberally in the letters he sent to his constituents. I know because I used to type them.