The main thing is to keep the main thing the main thing.
The following letters from Śrīla Prabhupāda make clear to his disciples what is the main thing for those who wish to practice and teach devotional service:
The main thing is that you must set nice example for the others by following very rigidly the regulative principles, such as chanting sixteen rounds on the beads, reading the books, getting up to attend maṅgala-ārātrika, etc. In this way everyone shall be encouraged.
And on the very same day, he wrote to another disciple:
It is so very nice to hear how the book distribution is going on. This is our main business, distributing these books all over the world and thus turning the minds of the fallen souls towards Kṛṣṇa.
In his purport to Bhagavad-gītā (6.24), Śrīla Prabhupāda comments:
The yoga practitioner should be determined and should patiently prosecute the practice without deviation. One should be sure of success at the end and pursue this course with great perseverance, not becoming discouraged if there is any delay in the attainment of success. Success is sure for the rigid practitioner.
In the Kṛṣṇa consciousness movement, then, the main thing should be to rigidly practice devotional service and share the nectar of advancing in spiritual life with others.
Śrīla Rūpa Gosvāmī elaborates on this principle in The Nectar of Instruction (8):
The essence of all advice is that one should utilize one’s full time – twenty-four hours a day – in nicely chanting and remembering the Lord’s divine name, transcendental form, qualities, and eternal pastimes, thereby gradually engaging one’s tongue and mind. In this way one should reside in Vraja [Goloka Vṛndāvana-dhāma] and serve Kṛṣṇa under the guidance of devotees. One should follow in the footsteps of the Lord’s beloved devotees, who are deeply attached to His devotional service.
In summary, the essence of all advice, or the main thing, is to organize one’s life around a strict, serious, and sincere practice of devotional service with the aim of becoming sufficiently strong spiritually to turn conditioned souls toward loving service to Śrī Kṛṣṇa, especially by distributing transcendental knowledge through Śrīla Prabhupāda’s books.
Devotees derive unlimited benefits from adhering to the rigid practice of devotional service: (1) By developing all good qualities, they become attractive; (2) They develop a taste for all things related to Kṛṣṇa and for sharing that taste with others; (3) They develop a deep affinity for hearing and studying scripture; (4) They develop transcendental vision. All these qualities make for highly effective distributors of Śrīla Prabhupāda’s transcendental books. In short, the distributors become empowered – invested with spiritual power.