Mark Suster is one of the enterpreneurs (now VC) which I follow, and perhaps more clear have the ideas.

This picture is a slide of one of his presentations. It is simple: to be lean until your company reach the inflection point (after you begin to generate reveneus) and then (dependig you strategy) you can begin to fat…

Don´t forget it, if not, you become a “junkie” looking for money everyday…

PS. If you are interested to know more about Lean sturtups, I recommend you this book