From Bhagavad Gita 17.14-16
Bodily austerity means to always show respect, particularly to gods, priests, teachers, and the wise. It also means to be clean, simple, celibate, and non-violent. That is how religious effort is expressed through the body.
Verbal austerity means speaking the truth in a manner that is affectionate, helpful, and not unsettling. It also means voicing the recitations that are part of your studies.
Mental austerity means practicing self-control via emotional contentment, gentleness, silence, and improvement of one’s character.