You can't have your cake

The one I use the most is "until the cows turn blue"

My most commonly used: "Does the pope shit in the woods?"

No sense beating around a dead bush
.
Take a random stab in the back
.
You can catch more flies with honey than a stick
.
A bird in the hand is better than a chip on your shoulder
.
You can lead a horse to water but you can't make him shit in the woods

This one time, at Band Camp, I stuck a feather in my cap.

Sorry I can only come up with my German favourite.
"Life ain't no pony-licking" (life isn't easy, mash-up between "life ain't no sugar licking" and "life ain't a pony ranch")

Pretty sure RuntimeErr0r is one of my former coworkers. He used to give me a lot shit for the time I said that thing about burning bridges…