"A good plan, vigorously executed today, is better than a perfect plan executed next week."

"A good plan, vigorously executed today, is better than a perfect plan executed next week."  This statement puts planning and execution in proper balance.  A flawed plan can achieve success if executed well.  Why is this so?  First of all because all plans are flawed in some way... no amount of planning will ever be able to account for all risks and uncertainties.  Second, acting gains the initiative rather than reacting to circumstances....your actions actually help create the future. Third, at some point in the planning process the leader will have gathered all the pertinent data relevant to the decision, the facts bearing on the problem,  considered all the alternative courses of action, and taken action to mitigate known risks. It is time to act. 
Failure to act, ask for more data, or consult more widely are all symptoms of  "analysis paralysis"...that set of circumstances where the leader misses opportunities due to their inability to act. 
To be clear, it takes courage as a leader to act under conditions of uncertainty and today's business environment has never been more uncertain.  The leader must overcome his fear of making a mistake, of being accountable for a bad outcome, recognize that no plan is perfect and be prepared to adapt during execution.