The sphere of concern vs. the sphere of influence

The 7 Habits of Highly Effective People by Stephen R. Covey is a classic book on self improvement. My favorite take away from the book is the sphere of concern vs. the sphere of influence (or control). If you are concerned about things you have no control or influence over then you will be spinning your wheels needlessly.  By focusing on what you can control or influence you increase your ability to be effective.