I highly recommend Getting Things Done. It's a whole system, but there are lots of small rules you can extract and use to great effect.

For instance:

- If an email takes 2 min to answer, do it.

- If it requires an action from you, but at another time. Move it to a "next action" folder (that you'll work through when you have the time).

- Also "archive" and "references" folders are useful, possibly organized by project.

- "Inbox zero" to reduce stress and keep yourself sane.