Whether or not your organization currently has a blog in place, your first job is to determine if you need one. Not every business will benefit from a blog because not every business has something they want to tell people three times a week...or even twice a month. If yours is that business, acknowledge it and move on to a form of marketing that better serves your site.
As you seek to discover whether or not your organization can use a blog to help achieve its goals, you may find yourself coming to a determination of the blog's purpose. It's difficult to say what will matter most in the success of your blog, but without a clear, maintainable purpose, your blog will almost certainly not achieve greatness.
Regardless of how simple or silly some of the most popular blogs in the world seem to be, you can be sure that there is a clear purpose behind each one.