The most common mistake by far is on profits. Startups that grow don`t produce profits. Investors make money on valuation increases, not profits. Real businesses rarely produce more than single-digit profits. Big profit projections are sophomoric. Take all those profits and dump them into marketing expenses and you`ll be better off.
Later, as the investment process proceeds (if it does), the latest regularly-revised plan will serve as a companion piece to the pitch and a key document for due diligence.
When writing a business plan, have an end goal in mind. You need to ask yourself, Where do I want it to go? What will my business look like? Stated differently, how do you want your business functioning in the next five to ten years? This is your vision.
But in real life, the plan related to seeking investment is a subset, a special case. Most real business plans are about managing a business and need a lot less description and research than the business plan related to seeking investment. In most cases, a lean business plan fits the business purpose best.
The more you test and review elements of your plan, the better your plan and business will be. This can save you from spending days developing a strategy that just isn`t feasible.
The business plan is a necessary but not sufficient condition for finding outside investors. The plan describes the business and what it might become, and that`s all. A beautifully written, edited, and formatted business plan will not make a less investible business more investible.