Inspired by the Most Hated Idioms thread, how about favorites?
One I’ve always liked is: “Even a clock that doesn’t run is right twice a day.”
You miss 100% of the shots you don’t take.
“I yam what I yam” - Popeye
“You can’t have your Crown of Thorns AND your 30 pieces of silver”.
Cogito ergo spud.
My old friend Jim used to say,“Ain’t that some shit?”
A saying that can be used in many situations.
“He’s a couple fries short of a happy meal.”
“He couldn’t pour piss out of a boot with instructions on the heel.” Kind of gross, but the picture is so absurd I have to chuckle.
“As drunk as a Lord”
“As pissed as a newt”
“Three sheets to the wind”
“Brahms and Liszt”
all have the same meaning but put it a little more creatively than just “hammered”.
When describing someone who’s a bit nuts, my old dad used to say, “She’s (or he’s) not wrapped too tight.”
He’d also say, when finished with something (it could be reading an historical marker, or at the end of a movie, dinner, or even when leaving a site like the Grand Canyon), “There’s your deal for you.” He always appreciated things, and loved learning about and seeing new things, but I always got the impression he was hard to impress. Saying “there’s your deal for ya” said to me he was satisfied, but underwhelmed.
My all-time favourite: “Been there, done that, got the t-shirt.”
“Well that’s better than a poke in the eye with a hot stick.” similar to: “Well that’s better than a kick in the ass with a frozen boot.”
From the Dukes of Hazzard, Uncle Jesse: “He’s as nervous as a long-tailed cat in a room full of rocking chairs.”
“Boy, are you in the wrong church”–Whenever anyone makes a statement about me that isn’t true.
“Shoot them all and let God sort them out”
“Leading the lemmings to the light”
And its corollary: “I sweet potato what I sweet potato.”
Anyway, one of my faves, which I learned in med school: “You can see a lot just by looking.”
Also: “Give a hoot, don’t litter.”
When I was a kid it was “Give a hoot, don’t pollute.” Made the rhyme.
I love the Internet Meme sayings:
“And not a single fuck was given that day.”
“Well THERE’S your problem.”
“Looks legit.”
etc. etc.
My all-time favorite is anything Foghorn Leghorn says.
“Nice boy, but about as sharp as a sack of wet mice.”
Whoosh!
My favorite: “We have met the enemy and he is us!”
By Walt Kelly, of course… 
My newest one, because of a true story a friend of mine told me.
“don’t build your hammock over the horseshit.”
That one’s actually as “old as dirt” (I’ll do you a favor and not link to TVTropes). It goes back to some Babylonian general (IIRC, it’s been a long time since I read about it) who said “Kill them all. The Gods will recognize their own.” Which is, of course my favorite answer for the OP.
“Barking up the wrong tree.” Vividly paints a funny picture which metaphorically matches the situation perfectly.