A person in Kṛṣṇa consciousness is always transcendental to the material modes of nature. He has no expectations for the result of the work entrusted to him, because he is above false ego and pride. Still, he is always enthusiastic till the completion of such work. He does not worry about the distress undertaken; he is always enthusiastic. He does not care for success or failure; he is equal in both distress and happiness. Such a worker is situated in the mode of goodness.
(Bhagavad-Gita---18:26---purport).

The favorable conditions for discharging devotional service are that one should be very enthusiastic in serving the Lord. The Lord in His form of Śrī Caitanya Mahāprabhu wanted the cult of devotional service to the Lord to be preached all over the world, in every nook and corner, and therefore a pure devotee's duty is to discharge this order as far as possible. Every devotee should be very enthusiastic, not only in performing his daily rituals of devotional service, but in trying to preach the cult peacefully by following in the footsteps of Lord Caitanya.
(Srimad Bhagavatam---2:8:21---purport).

One must perform devotional service with great patience. One should not give up the execution of devotional service because one or two attempts have not been successful. One must continue. Śrī Rūpa Gosvāmī also confirms that one should be very enthusiastic and execute devotional service with patience and confidence. Patience is necessary for developing the confidence that "Kṛṣṇa will certainly accept me because I am engaging in devotional service." One has only to execute service according to the rules and regulations to insure success.
(Srimad Bhagavatam---3:29:16---purport).

If one is purified by following Śrī Caitanya Mahāprabhu's orders—that is, by chanting the holy name of Kṛṣṇa—one must certainly be eager to render service to the Lord. This is the test. When one engages enthusiastically in the Lord's service, it is to be understood that he is reaping the results of chanting the names of Kṛṣṇa and Hari.
(Sri Caitanya Caritamrta---2:16:188---purport).

One should accept this opportunity to return home, back to Godhead, very enthusiastically. Without enthusiasm, one cannot be successful. Even in the material world one has to be very enthusiastic in his particular field of activity in order to become successful. A student, businessman, artist or anyone else who wants success in his line must be enthusiastic. Similarly, one has to be very enthusiastic in devotional service. Enthusiasm means action, but action for whom? The answer is that one should always act for Kṛṣṇa—kṛṣṇārthākhila-ceṣṭā (Bhakti-rasāmṛta-sindhu).
(Nectar of Instruction).

Similarly, there are six positive items for advancing in devotional service. First, while one should not be enthusiastic to attain material achievements, one should be very enthusiastic to attain the perfectional stage of devotional service. This enthusiasm is called utsāha. A living entity cannot stop acting. So when he is forbidden to becomeenthusiastic about material achievements, he should at once be encouraged to be enthusiastic about spiritual achievements. Enthusiasm is a symptom of the living entity; it cannot be stopped. It is just like a powerful engine: if you utilize it properly, it will give immense production. Therefore enthusiasm should be purified. Instead of employing enthusiasm for attaining material goals, one should be enthusiastic about achieving the perfectional stage of devotional service. Indeed, enthusing His devotees in devotional service is the purpose for which Kṛṣṇa descends to this material world.
(Narada Bhakti Sutra).