My favorite from an old Maine cowboy -
“Numb as pounded thumb”
Or as he said it:
“Nummer’n a pounded t’um” 
Me: Hey dad, you don’t look well. How do you feel?
Dad: Like 10 pounds of shit shoved into a 5 pound sack.
He could talk the arm off a bronze monument.
Dumber than a sneaker full of shit.
Jumpy as a cat on a hot tin roof.
She can’t cook but she’s got something to eat.
One I heard today:
Don’t let the screen door hit ya where the Good Lord split ya
There had been harsh words about something or other, and the Baptist minister was invited by the church leadership to find a different ministry. He was nevertheless invited to preach one more time and kept his temper in check, finally giving his parting message: “I have sincere gratitude for the kindness that most of you have shown me. In keeping with the dignity of this house of worship, I have no harsh words for any others. I merely wish them to take notice of the sprig of mistletoe attached to my coattails.”
Another two:
My brother once said about my tenacity in arguments: “He could talk a dog off a meat truck.”
Also:
“He has more chins than a Chinese phone book.”
“Throw some glass in that pneumonia hole!” – Close the window
“Throw a board in the hole.” – Close the door.
“Slicker than hammered owl shit.” – Said of icy roads (but I always wondered just who did the research…) 
'Stop that sniveling. You sound likea a dying calf in a snowstorm." To a crying child (not very sensitive…)
There’s a (possibly self-published) series of books titled, “Texas Crude”, “More Texas Crude”, “Even More Texas Crude”… that an old boyfriend introduced me to. Some were very funny, some just VERY crude. But I found a fair number that I heard growing up in rural Idaho, so they definitely get around.
I also love Southernisms–
“Carry me up to the store.” – Give me a ride to the store.
“Give me a little, sugah, honey.” – Give me a kiss.
“She mayshed a bump on her face.” She squeezed a pimple.
“What are y’all doing?” – What are you doing?
"What are all y’all doing? – What is everyone doing?
I’m glad to see this thread revived–thanks for the laughs!
Sharp as a bowling ball
Not the brightest bulb on the Christmas tree
Older than God’s parents.
Uglier than homemade soap
Courtesy of my Grandfather:
Make a Maggot jump off a Gut Wagon - used for something gross.
She took out her Tits and Tittered - used after hearing a woman laugh.
The Bull Gear jumped the Heifer Shaft - used when something broke down.
Busier than a One Legged Man in an Ass Kicking Contest - used when busy.
“SHE WAS SWEATING LIKE A BLIND LESBIAN IN A FISH SHOP”
“Happier than a fag in Boystown”
-----just a saying that may not quite fall under this category but hey…
METH…DRIVING DOWN THE PRICE OF BLOW JOBS FOR YEARS!
Lower than a snake’s belly in a wagon rut.
Slicker than greased owl shit.
Cute as a speckled pup in a red wagon under a Christmas tree.
I totally mis-read this as “uglier than homemade soup” and had a whiskey-tango-foxtrot moment there.
He couldn’t scratch his ass with a wildcat in each hand.
“People in hell want ice water, too.”
(Said in response to an unreasonable or impossible-to-fill request.)
Raining pitchforks and hammer handles, or cats and dogs. Homelier than Adam’s off ox. Complicated directions: all around Robin Hood’s barn. Helpless as a hog on ice. Madder than a wet hen.
Deader’n a three time zombie.
Zombie Shombie.
Someone with few morals is “lower than shark shit”
Mad as a meat axe
Useful as an ashtray on a motor bike (or tits on a bull)
Ugly as a hat full of arseholes
(Ugly person has…) A head like a dropped pie (or a bashed crab)
Someone who was extremely promiscuous and not at all fussy would “fuck a black snake with a festered arse”
A lazy person “wouldn’t work in an iron lung” or was “too lazy to get out of bed to take a shit”
Dumb as dog shit
so cold it would “freeze the balls off a brass monkey”
Busy as a 1 armed paper hanger (or a one legged man in an arse kicking contest) would mean you are “flat out like a lizard drinking”
I’ll put in some more if I remember them