When the cows come home to roost.
Familiarity breeds. (just leave off the last word and still makes perfect sense).
We will burn that bridge when we get to it.
Do unto others before they do unto you!
A bird in the hand gathers no moss.
I don't have a dog in this manger!
Don't count your chickens to spite your face.
A stich in time keeps the space-time continuum from coming apart at the seams, and keeps the proper order of cause and effect, and keeps the universe moving toward increased entropy.