If you have a hunch something is not working, pull the plug on it.
And actually, we should regularly pull the plug on the things that take most of our time, and see which ones we are truly going to miss.
This is clearly really applicable when it comes to marketing tactics. If there is something you consistently put your budget behind, pull the plug on it for one month. What happens? When the impact on key metrics is zero (or close to it), you have a great candidate for costs savings.
Who knows where programmatic advertising would be if more companies would regularly pull the plug.