I don't like overused sports references, but dribbling is such a nice way to explain the importance of planning that I can't resist. Whether it's dribbling a basketball or a soccer ball, it's a matter of keeping your eyes up, watching what's developing around you on the court or the field, even while also managing the details of dribbling the ball right in front of you. You have to plan as you go.
This is a good way to explain plan-as-you-go business planning. You have a plan but you're also watching developments around you, revising the plan as assumptions change and opportunities arise.
What I particularly like about this idea is the sense of planning instead of just a plan. Dribbling is about constant motion, constant change, constant review and revision. So too, planning should not be constrained by the original plan.
I particularly object to the people who think planning is bad because having a plan means being locked in, constrained in your options, because you're following a plan. On the contrary, having a plan should make it easier to identity changing assumptions and to change that plan on the fly. That's plan-as-you-go planning.