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!
Top-level reflection: In the daily hurly-burly of competitive business, it's hard to turn your attention to the big picture, especially those parts that aren't directly related to the daily operations. You need to take time periodically to really think about your business--whether it's providing you and your employees with what you want, whether there aren't some innovative wrinkles you can add, whether you're getting all you can out of your products, your sales staff and your markets. Writing your marketing plan is the best time to do this high-level thinking. Some companies send their top marketing people away to a retreat. Others go to the home of a principal. Some do marketing plan development at a local motel, away from phones and fax machines, so they can devote themselves solely to thinking hard and drawing the most accurate sketches they can of the immediate future of the business.