CC Ādi 12.51

mana duṣṭa ha-ile nahe kṛṣṇera smaraṇa
kṛṣṇa-smṛti vinu haya niṣphala jīvana
Synonyms: 
mana — mind; duṣṭa — polluted; ha-ile — becoming; nahe — is not possible; kṛṣṇera — of Lord Kṛṣṇa; smaraṇa — remembrance; kṛṣṇa-smṛti — remembrance of Lord Kṛṣṇa; vinu — without; haya — becomes; niṣphala — without any result; jīvana — life.
Translation: 
“When one’s mind is polluted, it is very difficult to remember Kṛṣṇa, and when remembrance of Lord Kṛṣṇa is hampered, one’s life is unproductive.
Purport: 

A devotee should always be alert, keeping his mind in a sanguine state so that he can always remember Lord Śrī Kṛṣṇa. The śāstras state, smartavyaḥ satataṁ viṣṇuḥ: in devotional life one should always remember Lord Viṣṇu. Śrīla Śukadeva Gosvāmī also advised Mahārāja Parīkṣit, smartavyo nityaśaḥ. In the Second Canto, first chapter, of Śrīmad-Bhāgavatam, Śukadeva Gosvāmī advised Parīkṣit Mahārāja:

tasmād bhārata sarvātmābhagavān īśvaro hariḥ
śrotavyaḥ kīrtitavyaś ca
smartavyaś cecchatābhayam

“O descendant of King Bharata, one who desires to be free from all miseries must hear about, glorify and also remember the Supreme Personality of Godhead, who is the Supersoul, the controller and the savior from all miseries.” (Bhāg. 2.1.5) This is the summary of all the activities of a Vaiṣṇava, and the same instruction is repeated here (kṛṣṇa-smṛti vinu haya niṣphala jīvana). Śrīla Rūpa Gosvāmī states in his Bhakti-rasāmṛta-sindhu, avyartha-kālatvam: A Vaiṣṇava must be very alert not to waste even a second of his valuable lifetime. This is a symptom of a Vaiṣṇava. But association with pounds-and-shillings men, or viṣayīs, materialists who are simply interested in sense gratification, pollutes one’s mind and hampers such continuous remembrance of Lord Kṛṣṇa. Śrī Caitanya Mahāprabhu therefore advised, asat-saṅga-tyāga — ei vaiṣṇava-ācāra: a Vaiṣṇava should behave in such a way as to never associate with nondevotees or materialists (Cc. Madhya 22.87). One can avoid such association simply by always remembering Kṛṣṇa within his heart.