Today and yesterday I tackled my Gmail inbox: at least 6600 messages from the past eight years.
Most mornings when I wake up, I am met by four to ten mass emails. Most of them are from mailing lists, scheduled to be sent at midnight somewhere in the world. These are things that I've signed up for, that I want to be aware of (programming news, social network digests), but that I don't need with any sort of urgency. I can skim through in batches once every week or so without missing anything. So I set up filters to review them at my leisure.
A vast majority of the work entailed purging old mailing lists, followed by labeling and archiving emails with "Friends" or "Family", "School" or "Jobs", among others. My taxonomy evolved as I sorted, and eventually I was left with a solid core of emails that were more difficult to categorize. Some of them prompted new labels (or the broader application of an existing label), while others simply didn't belong in Gmail. The most numerous of these were assorted messages to myself. I moved a few links to Evernote. I typed up a scan of a passage from a book that I liked. I merged a couple college essays into my school archive directory.
All but three emails have been either archived or deleted. Success!