More on saying "no" -- your schedule, time, life
It's a well-worn truism that great product managers, perhaps more than other disciplines, need to know how and when to say "no". Typically, this "mandate to negate" is discussed in the context of product scope. Indeed, telling stakeholders (not just customers) that they won't get what they want when they want it is difficult.
There are other areas in which "no" is just as important. Your schedule; your time; your personal life.
Read more in "Being an effective product manager means saying "no" to more than scope," a great piece by Greg Prickril.