Sri Krishna, advises Arjuna to constantly keep his mind fixed on Him and at the same time perform his Swadharma (to fight). The mind is purified when a man performs his own dharma, regarding himself as an instrument of God. Only a pure mind can constantly remember God.

CONSTANT PRACTICE IS NECESSARY TO REALIZE GOD

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Tasmat, therefore; anusmara, think of; mam, Me, in the way prescribed by the scriptures; sarvesu kalesu, at all times; and yudhya, fight, engage your-self in war, which is your own (caste) duty. Asamsayah, there is no doubt in this matter; that arpita-mano-buddhih, by dedicating your mind and intellect; mayi; to Me; esyasi, you-you who have thus dedicated our mind and intellect to Me, Vasudeva-will attain; mam eva, Me alone, as I shall be remembered. [When the Lord instructs Arjuna to think of Him, and at the same time engage in war, it may seem that He envisages a combination of Knowledge and action. But this is not so, because when one thinks of all actions, accessories and results that come within the purview of the mind and the intellect as Brahman, it is denied that actions etc. have any separate reality apart from Brahman. Therefore no combination is involved here.]Besides,