Here is a great article on the best practices that will allow you to more effectively deal with Outlook (and e-mail in general, whether you are on Yahoo! or Gmail).
I've been using this system, or a variation of it, for about 6 years now, and I can tell you that I go home every day with an empty Inbox.
Increase your productivity, while decreasing stress. Trust me, this process works. No longer do you have to let email manage you, you can manage it.
Related to this process, you must be using the Rules Wizard to help manage the incoming items. The power of that feature is almost endless.
Here is another great article on tips and tricks for using Outlook more effectively.