When I first signed up for Gmail, I totally drank the Kool-Aid and took Google's advice: don't delete anything. I had 1 GB of room, so why would I ever need to? Recently that's gone up to about 3GB, and my usage went to 511 MB, or 17%. That's after a little over three years of activity. So at my current level of growth:
17% / 3 years = 6% a year
It would still take me another 17 years or so to fill it up if my math is correct.
But part of me still thought I'd better start tackling the problem now. But what to delete? The thing I love about having all my email archived is looking back at cute conversations I had with my wife when we were still dating, funny threads with my friends, and reference items (more about that later). It's a nice little 'life record' that I think will be really neat to look back on twenty or thirty years from now (if the Internet still exists and doesn't somehow exist in our brains or something.)
So what do I go after? Looking through my mail recently, a lot of what I get is what's now known as "bacn". Bacn is not quite spam, but not quite real email either. It's notifications from Facebook, newsletters from companies, weekly flyers I've signed up to, etc. I even extend it to announcements of plays and other events that friends are putting on that are elapsed. I'm really never gonna wanna see this stuff when I'm old. It's like hoarding junk mail.
So I came up with a pretty good method of deleting. Basically, I started going through my box, one by one. As soon as I came across some bacn, I just did a search for the sender's email address (in Facebook's case, just "facebookmail.com") then did a 'select all' in gmail, and hit delete. Then, back to the inbox for more.
It took me about half an hour to do this, going back about three months of emails hunting for bacn. And in that time I deleted 1125 'bacn' emails. Holy shit! I emptied the trash to see how it would affect my space usage, and it had dropped from 511 to 472. That's 30MB cleared in 30 minutes! Nice.
Aside from the space saved, the other good thing about keeping your mailbox free of bacn is it actually makes it so much easier to look at the nostalgic old emails from friends or reference items I want to retrieve later.