Unless you are starting up a new site you probably already have a lot of content that deserves a thorough analysis and review. By taking the time up front to assess what you already have you can avoid the overhead that it will take to maintain outdated content long after it's expiration date.
Who cares?
It's an obvious question but the answer will help you decide what to do with your content. Maybe some content just needs a bit of an update to make it appealing to a new audience.
How can I sort it?
A content matrix is a handy tool to sort and prioritize existing content. If you flag something with medium quality ask yourself why?
Before the wireframe
Once you have reviewed your existing content and have an idea of each piece or type of content consider how you would like to group the information. How are the pieces of content inter-related? Have you considered content categories or a taxonomy structure? A well formed taxonomy structure will not only help you in planning how to group the content, it will help your end users who will use your site to find what they are looking for.
Look for holes
Most web sites have a the standard Contact Us and About Us pages. With a content inventory template you can summarize your exisiting structure to look for gaps.
Food for thought
By taking the time to assess what you already have in your exisiting system you will avoid duplication or ommission of some of your most valuable assets.
Your data!