What is discipline, as lord Krishna describes it in the Bhagavat Gita?
Krishna says in Chapter 12 of the Gita (Swami Nikhilananda translator):
Fix your mind on Me alone, rest your thoughts on Me alone, and in Me alone you will live hereafter. Of this there is no doubt.
If you unable to fix your mind steadily on Me, O Dhanajaya, then seek to reach Me by the yoga of constant practice. [constant practice means when your mind wanders away from thinking of the Lord, bring it back to thinking of Him. practice, practice, practice]
If you are incapable of constant practice, then devote yourself to My service, For even by rendering service to Me you will attain perfection.
If you unable to do even this, then be self-controlled, surrender the fruit of all action, and take refuge in Me.
And in 9.34 Krishna says:
Fix your mind on Me, be devoted to Me, sacrifice to Me, bow down to Me. Having thus disciplined yourself, and regarding Me as the Supreme Goal, you will come to Me.