Ideally, after writing marketing plans for a few years, you can sit back and review a series of them, year after year, and check the progress of your company. Of course, sometimes this is hard to make time for (;there is that annoying real world to deal with);, but it can provide an unparalleled objective view of what you've been doing with your business life over a number of years.
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!