KISS is an abbreviation of:
Keep It Stupid Simple
or:
Keep It Simple, Stupid

This principle has been a key, and a huge success in my years of software engineering. A common problem among software engineers and developers today is that they tend to over complicate problems.

Typically, when a developer is faced with a problem, they break it down into smaller pieces that they think they understand and then try to implement the solution in code.
I would say 8 or 9 out of 10 developers make the mistake that they don't break down the problem into small enough or understandable enough pieces. This results in very complex implementations of even the most simple problems, another side effect is spaghetti code, something we thought only BASIC would do with its goto statements, but in Java this results in classes with 500-1000 lines of code, methods that each have several hundreds of lines.
This code clutter is a result of the developer realizing exception cases to his original solution while he is typing in code. These exception cases would have solved if the developer had broken down the problem further.

• You will be able to solve more problems, faster.
• You will be able to produce code to solve complex problems in fewer lines of code.
• You will be able to produce higher quality code.
• You will be able to build larger systems, easier to maintain.
• Your code base will be more flexible, easier to extend, modify or refactor when new requirements arrive.
• You will be able to achieve more than you ever imagined.
• You will be able to work in large development groups and large projects since all the code is stupid simple.

There are several steps to take, very simple, but could be challenging for some. As easy as it sounds, keeping it simple, is a matter of patience, mostly with yourself.

• Be Humble, don't think of yourself as a super genius, this is your first mistake.
• By being humble, you will eventually achieve super genius status , and even if you don't, who cares! your code is stupid simple, so you don't have to be a genius to work with it.
• Break down your tasks into sub tasks that you think should take no longer than 4-12 hours to code.
• Break down your problems into many small problems. Each problem should be able to be solved within one or a very few classes.
• Keep your methods small, each method should never be more than 30-40 lines.
• Each method should only solve one little problem, not many uses cases.
• If you have a lot of conditions in your method, break these out into smaller methods.
• Not only will this be easier to read and maintain, but you will find bugs a lot faster.
• You will learn to love Right Click+Refactor in your editor.
• Keep your classes small, same methodology applies here as we described for methods.
• Solve the problem, then code it. Not the other way around.
• Many developers solve their problem while they are coding, and there is nothing wrong doing that. As a matter of fact, you can do that and still adhere to the above statement.
• If you have the ability to mentally break down things into very small pieces, then by all means, do that while you are coding. But don't be afraid of refactor your code over and over and over and over again. It's the end result that counts, and number of lines is not a measurement, unless you measure that fewer is better of course.
• Don't be afraid to throw away code. Refactoring and recoding are two very important areas.
• As you come across requirements that didn't exist, or you weren't aware of when you wrote the code to begin with you might be able to solve the old and the new problems with an even better solution.
• If you had followed the advice above, the amount of code to rewrite would have been minimal, and if you hadn't followed the advice above, then the code should probably be rewritten anyway.
• And for all other scenarios, try to keep it as simple as possible, this is the hardest behavior pattern to apply to, but once you have it, you'll look back and will say, I can't imagine how I was doing work before.

There are many, and I will look for some really great one to post here. But I will leave you with the following thought:

Some of the world's greatest algorithms are always the ones with the fewest lines of code. And when we go through the lines of code, we can easily understand them. The innovator of that algorithm broke down the problem until it was so easy to understand that he/she could implement it.

Many great problem solvers were not great coders, but yet they produced great code!

Absolutely not, it applies to many other programming languages and extends to many other areas in your life.
The areas that the principle doesn't apply to are: emotions, love and most importantly, your marriage

Author: Filip Hanik (The Apache® Software Foundation)
Source: https://people.apache.org/~fhanik/kiss.html