Captured thinking: You don't allow your financial people to keep their numbers in their heads. Financial reports are the lifeblood of the numbers side of any business, no matter what size. It should be no different with marketing. Your written document lays out your game plan. If people leave, if new people arrive, if memories falter, if events bring pressure to alter the givens, the information in the written marketing plan stays intact to remind you of what you'd agreed on.
Firms that are successful in marketing invariably start with a marketing plan. Large companies have plans with hundreds of pages; small companies can get by with a half-dozen sheets. Put your marketing plan in a three-ring binder. Refer to it at least quarterly, but better yet monthly. Leave a tab for putting in monthly reports on sales/manufacturing; this will allow you to track performance as you follow the plan.