Here's one I found yesterday that is useful for the poets amongst us when the poem won't come out the way you want it:

"By God," quod he, "for pleynly, at a word, / Thy drasty rymyng is nat worth a toord!"
[Chaucer, Geoffrey. The Canterbury Tales "Sir Thopas" l. 929]

And a much more modern one I've been using on difficult & demanding students expecting instant gratification this past week:

"You can't always get what you want". I'm sure you all know the source of that one.