Here are a few I can think of. Some are regional and (as Subvet and I found out) have different meanings in different parts of the country. Add your favorite ones in the comments.
Burning the candle at both ends
Talking out of both sides of your mouth
Being out of pocket
(being unreachable for a period of time) - this is a Southern meaning
Raining like a double-bladdered cow peeing on a flat rock
(a famous Subvet-ism)
Farting like a Blue-nosed mule
Drunker than Cooter Brown
Dead as a door nail
(Dickens does a whole monologue on why a door nail would be considered dead in the intro to A Christmas Carol)
Useless as tits on a teddy bear
(another Subvet-ism...I'm thinking MILITARY)
Neatest thing since sliced bread
(I still haven't figured out what's so neat about sliced bread!)
Cute as a button
(buttons are cute???)
Full as a tick
Busier than a) a one armed paper hanger or b) a one legged man in a butt kicking contest
Snug as a bug in a rug
(got a shoe??)
Sounds like you're a)wrestling alligators or b)juggling chainsaws right now
I've got to pee like a racehorse
(having seen a horse pee...I can understand this one.)
I'll be there, Good Lord willin' and the creek don't rise.
(I'll make every effort to attend)
Dumb as a box of rocks.
Couldn't pour pee out of a boot if the instructions were written upside down on the heel.
Hello, I'm so glad for you to see me today.
(common South Louisiana greeting)
Not the sharpest knife in the drawer
Now let's see what you come up with. I don't mind off-color humor..but let's try not to be just plain tacky ok?