If you don't write down your plan, don't ask anyone for money, time or effort to support it. (If you're funding it yourself, you can do whatever you want, of course.)
While no plan ever survived the first encounter with the enemy, trying to compete successfully in today's market takes conscious competence. So writing down your business plan lets you challenge your assumptions, and avoid costly mistakes - at least the ones you can anticipate.
The Rest of the Story...
There are lots of formats for creating marketing plans, sales plans and business plans available on the Internet, and we can help. The one you choose should just insure that you don't miss any critical variables.
After all, you know how to spell "assume", right?