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Śrī Īśopaniṣad
can easily be obtained by constant contact with the Personal feature of the Absolute Truth. Devotional service to the Lord consists essentially of nine transcendental activities on the part of the devotee:
1. hearing about the Lord, 
 2. glorifying the Lord, 
 3. remembering the Lord, 
 4. serving the Lotus Feet of the Lord, 
 5. worshiping the Lord, 
 6. offering prayers to the Lord, 
 7. serving the Lord, 
 8. friendly association with the Lord, 
 9. surrendering everything unto the Lord.
These nine principles of devotional service, either in sum total or one by one, can help the devotee to be constantly in touch with God, and therefore at the end of life it is easy for the devotee to remember the Lord. By adopting only one of these nine principles it was possible for the following renowned devotees of the Lord to achieve the highest perfection:
1. By hearing only, Mahārāj Parīkṣit, the hero of Śrīmad- Bhāgavatam , achieved the desired result. 
 2. Just by glorifying the Lord, Śukadeva Gosvāmī, the speaker of Śrīmad- Bhāgavatam , achieved his perfection. 
 3. By praying, Akrūra achieved the desired result. 
 4. By remembering, Prahlāda Mahārāj achieved the desired result. 
 5. By worshiping, Pṛthu Mahārāj achieved perfection. 
 6. By serving the Lotus Feet of the Lord, the Goddess of Fortune, Lakṣmī, achieved perfection. 
 7. By personal service to the Lord, Hanuman achieved the desired result. 
 8. By friendship, Arjuna achieved the result. 
 9. By surrendering everything that he had, Mahārāj Bali achieved the desired result. 
 
 Actually, the explanation of this  mantra  and of practically