step 1. Make good use of the Delete key.
I get tons of emails daily on the current project, if thee's any email thet gets sent that I know, or there's a reply later that includes the previous email - it goes to trash. Similarly, any that I read and think "yep, that's not my problem" goes to trash. If its important they can send it again :)
step 2. folders.
Sometimes you get emails you might need later, but don't care about right now. Move it to a folder and forget about it.
step 3. reply now, not later.
If you get an email that does requrie attention, if you can deal with it quickly, do so. Right now, immediately. Then you can delete it.
I still have nearly 300 emails in my 'todo' inbox, but you should see my deleted folder. If I didn't deal with them as above, I'd be even more swamped.