Are there official guidelines for mortifying the flesh in the various Christian denominations? Are there official guidelines for mortifying the flesh in the various Christian denominations? I'm interested primarily in the modern and historical practice exercised in Catholicism.
From the Code of Canon Law:


  Can. 1249 All Christ's faithful are obliged by divine law, each in his
  or her own way, to do penance. However, so that all may be joined
  together in a certain common practice of penance, days of penance are
  prescribed. On these days the faithful are in a special manner to
  devote themselves to prayer, to engage in works of piety and charity,
  and to deny themselves, by fulfilling their obligations more
  faithfully and especially by observing the fast and abstinence which
  the following canons prescribe.
  
  Can. 1250 The days and times of penance for the universal Church are
  each Friday of the whole year and the season of Lent.
  
  Can. 1251 Abstinence from meat, or from some other food as determined
  by the Episcopal Conference, is to be observed on all Fridays, unless
  a solemnity should fall on a Friday. Abstinence and fasting are to be
  observed on Ash Wednesday and Good Friday.
  
  Can. 1252 The law of abstinence binds those who have completed their
  fourteenth year. The law of fasting binds those who have attained
  their majority, until the beginning of their sixtieth year. Pastors of
  souls and parents are to ensure that even those who by reason of their
  age are not bound by the law of fasting and abstinence, are taught the
  true meaning of penance.
  
  Can. 1253 The Episcopal Conference can determine more particular ways
  in which fasting and abstinence are to be observed. In place of
  abstinence or fasting it can substitute, in whole or in part, other
  forms of penance, especially works of charity and exercises of piety.


One's confessor should be aware of any fasting or mortification beyond this, as one can be too ascetic, which might be a sign of pride.

(source; cf. this)