This 2-minute video from the Harvard Business Review (HBR) breaks down four factors critical to the success of business plans. This video is based on the article How to Write a Great Business Plan by William Sahlman. Full-text of the article is available via the library's subscription to HBR.
An easy-to-read blog post from HubSpot (a marketing firm) that details what a business plan should look like and provides examples. Keep in mind that HubSpot is a for-profit website so not all linked resources will be free.