Self-discipline is the master key to effectiveness in sales and any other endeavor you may choose. It's the ability to keep the commitments that you make to yourself, as well as the ability to delay gratification. It's also the foundation of good character.
Whenever I speak about the attributes that all successful people seem to share, someone always wants to raise the issues of honesty, integrity or courage. But these traits can more accurately be seen as a subset of self-discipline.
Honesty. The ability to be honest requires the self-discipline to tell the truth, even when the truth will hurt you.
We are built to avoid pain and seek pleasure. Because being honest can cause you pain, it requires that you act in spite of personal pain or the risk of loss. It's easy to want to retreat from a difficult conversation. It's easy to want to avoid telling the truth, especially when something is your fault or may damage your relationships.
Self-discipline is what allows you to be honest when it isn't easy to do so. That honesty—your ability to deal with the uncomfortable—can make you someone worth doing business with.
Integrity. It's tough to always walk the walk. But that's what integrity is. It means that your word is your bond and that you can be counted on.
It takes self-discipline to always do what needs to be done when it needs to be done. Sometimes it's difficult. Sometimes you just don't feel like it. Oftentimes distractions can derail you from what you planned to do. Self-discipline is what allows you to keep your word and to walk your talk.
Courage. Courage isn't the absence of fear. It's taking action even though you are gripped by 
Delayed Gratification. Of all the things that self-discipline enables, the most important may be the ability to delay gratification. It gives you the ability to trade a lesser reward now for a greater reward later.
