Maintaining the faculty of reason.
To put this another way...preventing tilt, or at least identifying tilt when it is happening.
The best players are going to be the ones who have studied the game, have direct experience with playing a winning style, and have the ability to keep emotions out of their head so they can make decisions rationally.
This isn't just poker advice either, philosophers dating back to antiquity have been telling us to control our emotions and strengthen our minds. Think about it for just a brief moment, if you were able to always think rationally (100% of the time) you would never have anxiety/fear/worry/etc.
Make it your goal (in poker and life) to strengthen your mind so you may always maintain the faculty of reason.