I seem to get about 200 work emails / day, and in my personal email, maybe 150. Much of it is unimportant. Some of it is urgent. My Inbox Zero strategy is about marking the important items with a star, doing the urgent items right away, and archiving the rest: A bit like the Eisenhower box (from Wikipedia):
I make a fairly modest use of labels, largely because most of my email is forgettable and I can usually find what I want with search. For almost all of my labels, I bypass the Inbox. I figure that if I have a label for it, I can rely on the visual cue (label name bolding) that there is something unread. I rarely delete email.
I'm doing this with GMail. The key feature I depend on is that I can set a checkbox for all of the email that I'm reviewing. It also helps that in GMail the checkbox selections are sticky, so that I can apply sequentially more than one operation to the same collection.
I quickly skim all of the subject lines for new mail in my inbox.
I decide if I'm going to archive more than I keep.
If I think I'm going to archive more than I keep, I select the checkbox indicating that I want to do something with all of the emails.
Then, beginning oldest to newest, I check or uncheck each of the emails depending on the subject. If I think I'm archiving most, then if there's something "interesting," I will uncheck it. (And if I'm mostly keeping, I'll check it to indicate that I'm archiving.) If I see something that is "interesting" in this stage that I don't intend to read right away, I will star it. For me, "star" means: come back later. By the way, I start with the oldest so that if there's something urgent that's becoming stale I can respond to it right away (and I will typically respond to it during this phase).
Then I mark all checked emails as "read," and then I archive all checked emails. It is important that I mark the emails as "read" so that if an email comes in on a very old thread and it's buried in a long list, I can surface it with "label: unread" without what I would consider false matches.
At this point in the process, I have a small collection of emails that I think I need to actually read.
Now I will read these typically from newest to oldest. Why? Because I've already dealt with the urgent-but-possibly-stale messages, so the priority at this point is usually responding rapidly to my correspondents who wrongly think email is the right tool for quasi-conversational real-time communications. Sometimes I will tell them to use Flowdock, GChat, or text messaging but the effort is rarely worth it.
If there's an obvious action for the email, I do it. If it is "interesting" and can be handled later, I start it.
At this point I archive everything except for new email that has appeared since I began the effort to get to Zero.
Even if there are a couple of brand new emails, I consider this Inbox Zero.
In short: Starting with subject lines only: Important and urgent? Deal with it. Important but not urgent? Star it and come back later. Everything: Archive.
Periodically (about twice/day) I will go into the starred email and see if there's anything that I want to attend to. Every few days I will archive everything that is starred on the principle that if I haven't done anything with them, they are actually not important.
Since you got this far, here's the mod revival song "Your Side of Heaven" by Back to Zero.