Sort of what the OP wanted but not that quotable- Lazarus in reference to Slayton Ford in “Methuselah’s Children”, with the abrupt departure from Earth.:
“Go for broke” took courage and character that most people didn’t have. Don’t grab a toothbrush, don’t wind the cat - just do it!
Darn. Now I’ll have to read Time Enough for Love again. 
Just in case everyone missed it, see post #38, above. AB ID’d the quote and the article it was from.
Heinlein used the theme of the quote several times, but the “How To Be A Survivor” article is almost certainly what the OP was looking for.
And it can be found in Heinlein’s Expanded Universe collection.
Having been inspired to reread it, I wanted to report that in addition to the passage Fenris quoted, Door into Summer includes an even closer paraphrase to what the OP mentioned.
[QUOTE=Heinlein]
But he also used to say that a wise man should be prepared to abandon his baggage at any time. I wondered how often I was going to have to do it to qualify as "wise.’
[/QUOTE]