A system beats willpower.
THE BASIC APPROACH
Treat the inbox as a list of things needing action, not a filing cabinet. Anything not needing action should be filed, archived or deleted.
FOLDERS
A small number of broad folders beats a deep hierarchy. Most people need: Customers, Suppliers, Admin, and an Archive.
Searching is faster than browsing, so folders exist to get mail out of the inbox rather than to make it findable.
FILTERS
Send newsletters, notifications and receipts straight to folders. They are reference material, not tasks.
A ROUTINE
Process the inbox at set times rather than continuously. For each message: reply if it takes two minutes, otherwise note the action somewhere and file the message.
WHAT NOT TO DO
Leave hundreds of read messages in the inbox. The inbox stops meaning anything, and genuine requests are missed. Use the inbox as a task list without any other record. Tasks arrive by phone and in person too.
ON A SHARED MAILBOX
Agree the convention with colleagues, or it works for nobody.