A frequent response to the need to manage knowledge is to create another database, define a file-naming convention or a new directory in which to store data. It doesn't work. Instead, integrate knowledge management into your workflow using a search engine over your message archives.
The traditional filing approaches to knowledge management usually start in a flurry of activity. They are accompanied with the great relief of the person charged with finding a solution as having found one. The additional effort required then grates with the normal way of doing things and inevitably things get left out. The new repository is no longer (or never was) a worthwhile place to go to get the answers. People fall back into emailing or talking on the phone, more information bypasses the system, new initiatives start up and the data is as fragmented as ever it was.
Search engines acknowledge that information comes from multiple sources. No need for a central repository in which to put things. The need is for a central source from which to get things. In addition, search engines acknowledge that you want to pivot your data. This means sometimes you want the information by date, sometimes by subject, sometimes by who it was sent from or to. Any filing convention that uses just one of these is inevitably less than ideal.
Coupling search to messaging gives you a knowledge management system with minimal effort, a source of current and historical information that gets more valuable with time, without the problems of where or how to file information.