There are an inordinate number of websites offering free templates for writing a business plan. However they fail to explain to the entrepreneur the intent, or style the business plan should adopt.
Structure is important, but the key to writing any effective communication is to never forget your reader. Copywriters learn early in their careers never to alienate the reader. The reader is king. The reader's needs and wants are more important than those of the writer. The writer is only a conduit.
I knew some writers who were so fanatical about the reader, they kept a photo representation on their desk during a writing assignment - just so they didn't forget who they were speaking to.