In honor of Worldwide Invest Better Day, we at The Motley Fool are recapping our "13 Steps to Investing Foolishly" -- steps you can follow to become a better, more Foolish investor.

In the following video, Foolish analyst Rex Moore looks at Step No. 5: how to avoid the biggest mistake investors make. That mistake is focusing on the short term and losing your temperament.

With all the short-term noise out there, it's hard to take a long, patient view to growing your investments, but it's absolutely crucial to do so. Rex calls this the $10 billion mistake, because that's the amount that Warren Buffett, arguably the world’s best investor, estimates he lost by focusing on the short-term price movements that kept him from buying shares of Wal-Mart many years ago.

Investing is just as much about controlling your emotions as it is about finding great companies. You can develop this emotional control using the six steps highlighted in the video.

Click the big green button to join the thousands of people celebrating Worldwide Invest Better Day on Sept. 25!