If there is any one "secret" of effectiveness, it is concentration. Effective executives do first things first and they do one thing at a time.
Context
Drucker devoted Chapter 5 of The Effective Executive to the principle of concentration. He observed that effective executives ruthlessly abandon low-priority tasks and protect focused time for high-priority work. He argued that doing one thing at a time — not multitasking — is how knowledge workers achieve results. This principle was central to his broader argument that effectiveness is a learnable discipline, not an innate talent.