I've used the Junk E-mail Filter in Outlook 2003 for almost 3 years, and the amount of spam I've received in my Inbox has been close to nothing. It's a great tool, but like any tool it's not perfect. I still get fraudulent e-mail from time to time and I've had some messages from colleagues blocked. Over time, I've discovered some basic tips that have helped me use the filter more effectively. I hope these help you as well.
Let me know if you've found tricks that help you get more of the mail you want, and less of the stuff you don't want.
—Jason Kozleski