Have An Overall Vision : When writing your business plan it is really important to have an overriding vision of what your business is going to do, what it is going to be, and what you want to achieve. Very often it is tempting to get straight into the technical details, the monetary concerns, financial matters, where you will be sourcing supplies, etc. Now all these things will be vital in your business plan, but it has to be held together by a coherent, broader vision.
Research Your Competitors (But Don't Copy!) Every business plan should focus a lot on the business's potential competitors, because research and analysis of the competition effectively gives you plenty of useful information. It may guide you as to where you should be advertising and marketing, or certain strategies to use or ones to avoid because you see they have been used unsuccessfully by others.
A business that makes sense and is going to be sustainable in the future is one that has that clear vision within which all the smaller parts contribute to make it successful.
I often see people split into two camps. On one hand those who almost ignore competitors in their business plan, because they do not want to think about the issue yet and feel so confident they have a great idea for the market regardless. But I recommend not being overconfident when it comes to competitors. They are still there for a reason, they are still around and in business for a reason, so view them with that in mind.