Reducing the power of a bad karma
The Sankha Sutta is one of the best sutta's I know of that provides instructions on how to transcend and overcome the effects of past unskillful kamma. 

http://www.accesstoinsight.org/tipitaka/sn/sn42/sn42.008.than.html


  This is how there comes to be the abandoning of that evil deed. This
  is how there comes to be the transcending of that evil deed.
  
  ...when the awareness-release through equanimity is thus developed, thus
  pursued, any deed done to a limited extent no longer remains there, no
  longer stays there."