You want to have a well thought out plan to guide you so that during the short term any distractions or obstacles you may encounter will not divert you from your goals. However, you need to be evaluating and adjusting as often as necessary so that the end goal is always in site. Much like a ship crossing the ocean, you will set your coordinates for the desired destination. During the trip you may hit high seas and rough winds, but the initial plan will carry you through. As the rough seas subside you can reevaluate your position and make any adjustments as necessary.
The one thing that I have realized is that there is simply no magic formula to obtaining a perfect result in marketing. There are best practices that you can utilize to maximize the return on your investment, but every business has unique needs and goals. The media mix of what will work for one business may be entirely opposite of what works for another. This is why it is important to constantly measure and adjust so that you can continually learn what works best for your organization.