I tried to find the etymology of the cliche "when life gives you lemons, make lemonade" on the Internet, but so far I haven't had any luck. It won't even tell me if it's a maxim or not.
Dale Carnegie popularized the cliché, but I found it in print five years prior to Carnegie's first publication. This is from a sidebar of maxims in a men's clothing advertising periodical called Men's Wear, 1908/09:
In business turn obstacles into conveniences. When handed a lemon—make lemonade of it.
This reference most likely precludes Carnegie from being the phrase's progenitor as he was selling lard at the time of its publication.
It seems Carnegie did not use the phrase in print until his last of six books, published in 1948, How to Stop Worrying and Start Living, in which he has a chapter titled "If You Have a Lemon, Make a Lemonade" that ends with his Rule #6:
When fate hands us a lemon, let's try to make a lemonade.
Actually, the saying is originally attributed to Elbert Hubbard: "A genius is a man who takes the lemons that Fate hands him and starts a lemonade stand with them." (Reader's Digest, October 1927) ref
However, the saying is usually credited to Dale Carnegie, who published it as: "When fate hands us a lemon let's try to make a lemonade." (Rule #6, at the end of Chapter 17 in Carnegie's "How to Stop Worrying and Start Living" published in 1948)
EDIT: Probing a little further, I acquired the October, 1927 "Reader's Digest". As foretold, I discovered Hubbard's "lemon quote", right there on page 343, borrowed from "The Independent".