What is your favorite teaching from Stoic philosophy?

The Discipline of Ascent trains you to pay attention to your thought process and cultivate a healthy mind.  You must pause and think about your responses to life, rather than allowing instincts, habits, and default behavior to run your life.  Preventing negative thinking is an active process, you will improve with more practice. Most importantly, learning to guide your thought processes will allow you to put more energy into what you control.