An online group I'm involved with has been having some fun with regional phrases, or just colorful phrases they know and love. I know I've had projects that could best be described as being "like nailing Jell-O to the wall" (I have to remember that one!). Others being batted around are:
Like herding cats
That dog/puppy won't hunt
He could mess up a one-car funeral
He/she doesn't know come here from sic em!
Her face could curdle milk at ten feet!
It's higher than a cat's back
She's no bigger than a minute
He is wearin' me slick!
She is slower than honey in January
It's rainin' so hard the animals are linin' up two by two!
He's not the sharpest knife in the drawer
I love this one, too, from a Southerner: You can say anything, as long as you add "bless his/her heart" to it.
My favorite from growing up was my mom's reply whenever I couldn't find something right under my nose—"If it was a snake, it would have bitten you"—and my dad's advice to help me find something I lost: "Look in the last place you put it."