This is a very good advice I learned from Reader's Digest Book "Write Better, Speak Better".
I don't know how it originated, but many people use it everyday. I read in an article, saying that KISS is a guiding principle in the British Army, but I have not checked it. I learned it from a magazine on Business Management.
When I was young, I thought that "simple" things are not good, and I pursued after "high, deep" things. e.g. I spent more than 3 years reading one Mathematics book, page by page, theorem by theorem.
When I gained more experience, especially after reading some Maths book, which I thought to be too simple in the past (e.g. H. B. Hildebrand's "Advanced Calculus for Applications", W. Kaplan's "Ordinary Differential Equations"), I changed my view.
Writers who wrote plain, simple books have spent tremendous amount of time in arranging the material, so that the readers can master them easily, painlessly and logically. Writers who wrote books weighing "tons", usually just put their thoughts down, usually without checking them. And the readers have to struggle through these tangled thoughts.
Hence I advise people to choose books written by masters, (e.g. Maths book by Richard Courant, H.B. Hildebrand, W. Kaplan, S. Lang, Murray R. Spiegel, R.V.Churchill ....), and learn from the best.
Lord Jesus taught us to be on guard on "deep things of Satan" : "But to the rest of you in Thyatira, who do not hold this teaching (Note : teachings by a prophetess, Jezebel, about practicing immorality and of eating food sacrificed to idols), who have not learned what some call the deep things of Satan, to you I say,.. " [Revelation 2:24]
These are the guiding principles of Kroc, after he had purchased the patent from the McDonald brothers who invented the "factory production-line-method in food serving, at extremely competitive price.", and started opening up many restaurants. How well this was received may be seen by the popularity of McDonald restaurants in the years after he implemented this.
The Bible contains historial lessons of hundreds of years. What they have faced in the past will provide you guidance as to how to handle your own.