The purpose of the Marketing Plan is to identify and create competitive advantage. Marketing planning is the real form of the company to provide a strategic response to a changing global competition pattern consisting of: business size adjustments, changes in product and market scope, creation of new networking relationships with other organizations.
You can't do a marketing plan without getting many people involved. No matter what your size, get feedback from all parts of your company: finance, manufacturing, personnel, supply and so on--in addition to marketing itself. This is especially important because it will take all aspects of your company to make your marketing plan work. Your key people can provide realistic input on what's achievable and how your goals can be reached, and they can share any insights they have on any potential, as-yet-unrealized marketing opportunities, adding another dimension to your plan. If you're essentially a one-person management operation, you'll have to wear all your hats at one time--but at least the meetings will be short!