We're sure you've heard of the supplier scorecard. In fact, you probably use it yourself or your customers track your performance with it. A short article in esourcingforum.com makes some recommendations on improving the process.
A short article in the E-Sourcing Forum by Chris Pfauser does a concise job explaining how the determine if your processes are mature. Substitue "EDI" for "IT" as you read the story and it'll be even more relevant. If your shop is like most I've seen and worked in, your processes will probably never be completely mature, but if you head in the direction outlined in the article you'll be in good shape.