The first step was to start with success. I created a folder called old_mail and moved my entire inbox there. Now my inbox was empty and I was ready for Inbox Zero. I blocked time in my calendar to go through old unread messages.
The second step was to reduce the flow to my inbox. I created a ~holding folder and added rules that moved lower priority mail from mailing lists, etc. there. I already had some rules but now I ratcheted up their selectivity. I also decided that when I see non-solicited email I will immediately unsubscribe (which is not easy since I have multiple email addresses) and, just in case, mark it as junk.
The third step was to get some help efficiently implementing GTD so I downloaded the evaluation copy of the GTD Outlook add-in. The main benefit of the add-in is that it extends Outlook tasks with projects and contexts (@Call, @Computer, @Home, etc.). It can also manage delegation with reminders and a few other things. My biggest annoyance is the UI, which leaves much to be desired in terms of productivity, especially in the case of picking a calendar spot to defer a task to. Also, filing items in folders is rather slow.
So far so good. Inbox Zero is definitely better than what I was using before but I wish I had better software tools inside Outlook for it.