Further to my suggestions for keeping your inbox in check, Jordan Furlong has a few suggestions to add:

• Clients. Set up an extranet for each client; add an RSS feed. If you need to ask your client a question, call her.
• Colleagues (down the hall). Something wrong with your legs? / use the phone.
• Cooperation. To set up meetings, use Google Calendar or the like; instead of round-tripping documents, use Google Docs or similar; or set up an office intranet for a given file or matter.