We've noticed some common behaviors that people use to deal with email overload through our interviews. I see a combination of a few of these things, typically among people who get a lot of email.
This includes keeping/marking as unread, starring, or flagging emails. The reason people seem to do this is it visually distinguishes important emails from the rest. However, this doesn't work for some because they start flagging/starring everything-- or they don't go back and check on them.
2. Unsubscribing from everything
After some years of a lot of email, people get pretty tired of the junk (but not quite junk) mail that they receive. Discounts, offers, and notifications of cool events just don't make the cut for a number of people. People who don't do this get inundated in their inbox, but they do like seeing this extraneous information coming through. It just makes it harder to pick out the actual important emails-- thus, starring and the like.
Some that like the sort of extraneous information create filters for these emails to go straight into folders. It's a bit more work (though hitting unsubscribe can lead you down a rabbit hole as well) and sometimes things get filtered into that folder that oughtn't have--one of the biggest reasons why people stopped setting up filters into folders, actually.
3. Attempting Inbox Zero
Most people check their email constantly and make sure what's in their inbox is actually important. They're able to delete junk mail quickly enough, but then the semi-important emails pile up and it's like seasonal cleaning (if it hasn't overwhelmed them by then). A good number of people are pretty fastidious, but it seems a badge of honor that they've earned through training and time.
An interesting strategy for sure, but the exact opposite from the strategy above--unless it catches their eye, they'll just let it go. I've got to give kudos for not feeling as compelled to look at and respond to every email. Another prominent thought here is that if someone really wanted to get a response, they'll send it again.
5. GMail Important Markers
Very few people train their GMail to recognize important emails and triage that way. It seems like a good number of people just end up getting all their emails as Important. I did come across a few people who did spend a little time upfront to retrain their GMail and now find it useful.
6. Other plugins or products
And a good number of us are always searching for ways to help us with overload (besides the built-in GMail important marker). There a lot of great startups in this space (including us!) to try to take that load off of peoples' shoulders. You can check out Claire's infographic highlighting a few that attempt to take on and beat back email overload.