Reducing the power of a bad karma As we all agree none of us have a white past, we've all have done some bad thing in our past before coming to the path and in previous lives.

So how should we use the good karma we generate today to
reduce the impact of bad karma from previous lives and from our past?

I've heard that this is possible.
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."