If by the tongue you chant Hare Krishna mantra and by the tongue you taste prasadam, you will be perfect, simply by executing these two things. Do not accept anything, do not allow the tongue to touch anything which is not offered to Krishna. This is one item.Another item — engage the tongue always in chanting Hare Krishna. You become perfect.
Lecture on Bhagavad-gita 1.10 — London, July 12, 1973