How can we avoid waste creeping back into processes in the future?

Lean and similar methodologies focus on the elimination of waste in our processes and in our organisations.  An important first step to improving a process is to identify waste, which can take many different forms, some of which are easier to see than others. Yet, often addressing waste is a harder challenge--this requires us not just to eliminate the waste itself but also ask ourselves how did this process end up being 'wasteful' in the first place, and how can we avoid waste creeping back into the process in future?