Giving good feedback is key to a successful design project.
Regardless of what you're having designed - logo, website, Annual Report, brochure, etc. - your input is important and totally necessary for your project to be successful.
You're paying good money for your project and you want great results, but the onus is on you to give good feedback. After all, designers aren't mind readers. No one knows your business like you, and the magic happens when your knowledge comes together with your design team's skills and expertise.
Below are a few guidelines on how to provide effective feedback and, just as or more importantly, what not to do.