“You’ve gotta keep control of your time,” Buffett says, “and you can’t unless you say no. You can’t let people set your agenda in life.”
Fast Company has an article on Why Productive People Have Empty Schedules. Buffett and other leaders emphasize the importance of saying “no” in taking control of your schedule.
GTD emphasizes the value of having a complete and current inventory of your commitments. That way you’ll know what you can say yes to, and when saying no is more productive.