According to David Allen's Getting Things Done productivity system, a calm, undisturbed mind is reflected in an empty Inbox. I try and empty my email Inbox every day, by acting on each message and then filing it in a reference folder. This is where a piece of software called Categoriez saves my life every day.
When keeping track of ongoing conversations with camera manufacturers and PR companies, I need to unite messages from different sources under a single heading for each make of camera. This is where Outlook's categories function comes in.
Outlook allows you to assign a category to each Outlook item -- emails, contacts, calendar items etc. Once you have added Category to your columns (click on any column header > customise current views > fields > add categories), you can sort emails and contacts by category, just by clicking on the column header, same as with Received or From.
This great idea is frustratingly under-implemented -- which is where Aladdins Categoriez comes in. Categoriez is an excellent Outlook plugin that adds a sidebar to your Outlook window, making your Master Category List available on the fly.
When I receive an email from a PR contact, I reply, tick the relevant category in the sidebar, and use Ctrl+Shift+V to whisk the message into my PR Contact folder. Then, when I need to refer back to the message, I simply open up the PR contact folder, and there they all are, sorted by category.
Categoriez also allows you to filter your emails by category, putting the relevant messages at your fingertips. Once you get used to categorising everything, the system becomes trustworthy enough that you can empty your inbox every day, which is the essence of GTD. A calm, undisturbed inbox and a calm, undisturbed mind.