Will power. To practice, try going to sleep when you aren't tired. Try eating when you aren't hungry. Try staying awake when you want sleep. Try not eating when you are hungry. Practice, not once... Many times. Cast out unwanted thoughts. Cast out illness. Practice so to help rise above impulse... Practice because you have weighed the joy of things and see the means to peace and happiness. That is after all why we want discipline, no? Choice, not control. Choose, practice and have faith.
Love above all to free yourself from the suffering of thwarted expectations striving nonetheless towards the ideal. Realize what is in your power and what is not, own both.