Customer acquisition cost (CAC): How to calculate & reduce it

The best marketers in the world help their companies grow for the long term. However, periodically marketers seem to go through a wave of collective insanity where we forget about the ‘long term’ part of the discussion and start to chase growth at any cost. Whether it’s through inattention, ignorance, or excitement, marketers sometimes make very bad business decisions.
So, how can we protect ourselves from ourselves? How do we make sure we’re not just world-class marketers, but also world-class business people?