“I shall pass through this world but once.
If, therefore, there be any kindness I can show, or any good thing I can do, let me do it now;
Let me not defer or neglect it, for I shall not pass this way again.”
Now if only I knew who said it! Google searches turned up sources as various as Etienne Grellet, Mahatma Gandhi, and Marcus Aulerius--an almost 2000-year span.
But no matter who said it, it's a wonderful motto to live by.