Tomorrow is another day
The last line of the American Civil War novel Gone With The Wind:
Scarlett O'Hara: "Tara. Home. I'll go home, and I'll think of some way to get him back. After all, tomorrow is another day"
This joins "Fiddle-dee-dee!" and "Great balls of fire!" as lines spoken by Scarlett O'Hara that have become commonplace in the language.
See also: the List of Proverbs.