The Bhagavad Gita (17.14-16) describes a threefold austerity.
"Worship of the gods, of the twice-born, of teachers, and of the wise; cleanliness, uprightness, continence, and non-violence-these are said to be austerity of the body." (17.14)
"Words that do not give offense and that are truthful, pleasant, and beneficial, and also the regular recitation of the Vedas-these are said to be austerity of speech." (17.15) A well-known Sanskrit verse explains how to be truthful in speech: "Speak what is truthful, speak what is pleasant, but do not speak an unpleasant truth. Nor should you speak an untruth just because it is pleasant to hear. This is the way of eternal religion."
"Serenity of mind, gentleness, silence, self-control, and purity of heart-these constitute austerity of the mind." (17.16) According to Sri Ramanuja, serenity of mind refers to absence of anger, gentleness means being helpful to others, and silence means control of speech by the mind. A forced silence unaccompanied by mind control is not of much help.
According to the Bhagavad Gita, there are three kinds of austerity based on our inner nature: the inferior kind (tamasic) is practiced with a determination based on foolishness, by means of self-torture, or for the purpose of ruining another (17.19); the mediocre kind (rajasic) is practiced in order to gain respect, honor, and reverence, and for ostentation (17.18); the superior kind (sattvic) consists in the practice of the austerity of body, speech and mind with supreme faith by steadfast people, without the desire for results (17.17).
Every austerity needs to be judged by the strengthening effect it has on the will, and by the disciplining effect it has on the mind. We may undertake fasts and vigils on auspicious days, but they will not effect any spiritual transformation if simultaneously we do not keep the mind on a higher plane.
Our will is usually bound by desires, which sprout from our mental impressions. The will becomes free from desires and grows stronger only by repeated practice of spiritual discipline, thinking noble thoughts, and doing work as worship. We need to be steadfast with this practice no matter how hard our mind rebels against it. Strengthening of will requires practice and perseverance.