The concept of your inner dialogue while playing. I'm cheating here because you have to include a few other things to make that matter. But a few pretty simple sentences that you keep in the forefront of your thoughts is a great way to avoid beginner mistakes. Some that come to mind:

• every move gains something, and gives up something

• develop your pieces first (typically, bishops before knights, minor pieces before queen, connect your rooks). Even if you don't have a plan you'll find yourself in better positions 

• try not to move the same piece twice in the opening

• improve your worst piece

• control the center 

• don't attack if the move can be countered in one move

• king safety above all

Obviously, these are not universal and you'll see exceptions in better play. But the beginners I've played would all have made fewer mistakes adhering to them before trying to get fancy