My system is simple: do not sign-up for newsletter or notifications (e.g. Facebook, Twitter, GitHub, etc.) you won't read, unsubscribe anytime you realize you don't read the ones you receive, delete emails you won't ever need again, archive the ones that are nice to keep around but won't require anything from you anytime soon, and use filters whenever possible.
So in the end, I don't receive that many emails per day. I certainly don't receive any promotions in the mail… that's just asking for trouble.
Right now I have 25 threads in my inbox and the vast majority of them I keep around because they have pictures I want to save in iPhoto but haven't had time to. (I need to find a system for that actually)