About the single easiest thing you can do to keep on top of an email inbox is not to use it for long term storage.

The simplistic way of achieving this is to delete every message once it's been read. But the problem with this approach is that not every message can be deleted once it's been read. Suppose it's details of an order that you'll need to follow up on, or the contact details for a meeting that's going to take place next week? You'll need to keep these, at least for a while, but ideally you don't want them hanging around cluttering your inbox.

The solution is very straight-forward.

1. Create a new folder called "waiting-for", and pin it st the top of the folders list
2. When an email comes in, read it.
3. If it's not needed any more, delete it.
4. If it's relevant to something that you're waiting for, drag it into the "waiting for" folder
5. When the order arrives, or after the meeting, or whatever - delete the relevant message from the "waiting for" folder
6. Periodically scan through the "waiting for" folder for anything that's late, and delete anything that's aged out