From my aunt when I fretted about making a major life decision and how I’d be so old when it came to fruition:
“How old will you be if you don’t do it?”
I don’t think this is going to replace “head, shoulders, knees and toes” as a kiddie song. 
“Have fun NOW! You’ll be a long time dead!” (From an Aussie, of course!)
And,
“You can’t push a rope, dear.” (From my Gran, after complaining of an uncooperative husband.)
A tired dog is a good dog.
And
Be nice to the janitor.
“Hey! How about you worry about [kaylasdad99]?” – my dad, any time I complained that one of my siblings was getting better parental treatment than I thought he or she deserved.
“Don’t be such a c*nt”
Best bit of advice I ever got was from my attending, a pediatric neurosurgeon, during my residency rotation through the local Children’s hospital. A bit tired and cranky, I made an unmerited, flip comment about ancillary staff. His reply: “Qaddie, nobody likes a smartass”.
And from my old man: “Be good to yourself, you deserve it!”
A cleaning lady is cheaper than a divorce.
Sure, but it can be difficult to find a cleaning lady who is hot, single and willing to work in a French maid’s uniform.
And then there was George Best ( the British soccer player):
(Probably a paraphrase) I spent most of my money on booze and loose women. The rest I just wasted.
(me, wondering aloud nervously if the super tampon was right for me)
A friend’s mother: oh, honey, they all fit.
“Just don’t.”
Pithy advice, yes.
Good advice, well…
“Never trust a fart.”
Advice for the farter. Because it might turn out not to be one.
Never change lanes on a bridge.
The advice wasn’t “eat everything you can pronounce.” That would be pithy yet really stupid.
I’ve probably received a lot of pithy advice which I’ve forgotten. Something that I haven’t, was from a scraggly working class guy whom I ended up catching a ride with from central California to Portland when I was seventeen (long story). I was supposed to pay my share of the gas but I ran out of money buying food. So he kicked me out of the car and left me by the side of the road 20 miles from Portland. He leaned out the window as he drove off and said, “in the future, remember to keep the money straight.”
I’ve remembered.
Protect your brain - don’t get pithed
Dad used to say - almost always to me - “ya know why they don’t send donkeys to school?”
mmm
In Pennywise’s voice, right?
They ALL fit down there.
Indeed. Cyanide is pronounceable.