Writing a business plan doesn't mean you have to hole away in a library for months compiling 40-plus pages of text. You can create a plan in under a month, working part time. Use a presentation format like PowerPoint or Keynote to save time, and make it easier to share your plan.

Not all founders or start-ups are the same, of course. You'll want to plan in more detail if you're raising capital or taking on a lot of risk—like investing your savings, leaving a job, or supporting a family. Less detail is fine if you aren't raising money or taking on much risk. For example, if you are writing code in your dorm room, you can experiment to find out what consumers will latch onto before thinking about implementation or financing. 

But either way, you need a plan, and here's why:

1. To avoid big mistakes: The last thing you want to do is work on your start-up for a year, only to realize you were doomed to fail from the start. Many founders learn the hard way that they didn't set aside enough capital to reach their goals, took on partners with the wrong skills and resources, or don't have a viable way to make money. Developing and sharing a business plan can help ensure that you're sprinting down the right path.

2. To counterbalance your emotions: At times during your start-up experience, you'll be manic—so passionate about your ideas you lose sight of reality. At other times, you'll be overwhelmed by doubt, fear, or exhaustion. When your emotions get the best of you, having a business plan lets you step back, and take an objective look at what you are doing and why, what you know for a fact and what you are trying to figure out.

3. To make sure everyone's on the same page: Chances are, you are not building a company by yourself. Ideally, you'll have partners, so you can launch faster, smarter, and with less need to pay employees or suppliers. Even if you don't have partners, you'll have family, friends, and advisers involved. A business plan helps get everyone involved in your start-up heading in the same direction.

4. To develop a game plan: At a start-up, execution is everything. That means you have to set priorities, establish goals, and measure performance.  You also need to identify the key questions to answer, like "What features do customers really want?," "Will customers buy our product and how much will they pay?," and "How can we attract customers in a way that's cost effective and scalable?"  These are all things you'll address during the business planning process.

5. To raise capital. If you raise or borrow money—even from friends and family—you'll need to communicate your vision in a clear, compelling way. A good business plan will help you do just that. An October 2007 study by Babson College found that start-ups with a business plan raised twice as much capital as those without a business plan within the first 12 months.