Bg 12.8

mayy eva mana adhatsva
mayi buddhim nivesaya
nivasisyasi mayy eva
ata urdhvam na samsayah

Word for word: 
mayi — upon Me; eva — certainly; manah — mind; adhatsva — fix; mayi — upon Me; buddhim — intelligence; nivesaya — apply; nivasisyasi — you will live; mayi — in Me; eva — certainly; atah urdhvam — thereafter; na — never; samsayah — doubt.

Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
Just fix your mind upon Me, the Supreme Personality of Godhead, and engage all your intelligence in Me. Thus you will live in Me always, without a doubt.

Purport by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
One who is engaged in Lord Krishna’s devotional service lives in a direct relationship with the Supreme Lord, so there is no doubt that his position is transcendental from the very beginning. A devotee does not live on the material plane – he lives in Krishna. The holy name of the Lord and the Lord are nondifferent; therefore when a devotee chants Hare Krishna, Krishna and His internal potency are dancing on the tongue of the devotee. When he offers Krishna food, Krishna directly accepts these eatables, and the devotee becomes Krishna-ized by eating the remnants. One who does not engage in such service cannot understand how this is so, although this is a process recommended in the Bhagavad-gita and in other Vedic literatures.