Writing a Business Plan is key to both raising capital and running a company. A business plan is a document (typically ranging in length from 20 to 100 pages) that details the resources, history, strategy and tactics used by the company to help it grow. The basic premise is that if you plan properly, fewer things will surprise you. In other words, "plan the work and then work the plan."
If on the other hand, you are ambushed by changing market conditions, new competitors, fundamental changes in prices or costs, etc, you can react quickly and optimally by re-working the plan and using the new version to guide your actions. Many organizations revisit their business plan on an annual basis, often as part of their annual goal-setting and budgeting process. In some fast-changing industries (say, high tech) the business plans may be rewritten every fiscal quarter.
Here are some resources to help you write and update your business plan: