Before I start writing the business plan I should have done all of the homework relative to the industry, the market, the headcount commitment, the profitability, the capital costs etc...
Therefore when I prepare business plans I start with the writing of the executive summary. If I start with the executive summary I will cover all of the important points of the business plan to which I already did the research and it helps set the tone for the rest of the plan. However, keep in mind, just because I started the business plan with the executive summary doesn't mean I can't go back to it and fine tune it if I should learn something new about the industry or market.
and; remember, the executive summary is the most important part of the business plan and is likely the only section a potential investor reads! Even if a potential investor is interested in your plan and invite you for a presentation the executive summary is still probably the only section they read!
Read this PowerPoint presentation from a successful angel investor about the importance of an Executive Summary and to write it first.