I usually immediately read, respond to, and archive my emails with appropriate labels. The 15 or so I'll keep in my Inbox are usually emails that need me to take some action that I cannot do "immediately." Usually, to write a lengthy or thought-out response (so monthly emails from my faraway friends stay here), something I need to refer to quickly from my phone (the ticket pickup information for the ballet I went to last night), something I want to RSVP for but need to check my schedule first, etc.
After coming home to over 100 new emails after my week in Cancun, I narrowed it down to less than 30 pretty quickly and easily. But it's been at 27 emails for the beginning of this week. I just got it down to 17 and I feel SO SO good.