Smart Mailbox Formula: Flagged Messages
One very simple way to keep your email organized is by using flags in Apple's Mail program.
Use Smart Mailboxes to Organize Your Email
If you're anything like me, you tend to hold on to your email for months, years, or pretty much all of eternity. 
Now that I have things like Google Apps for Your Domain and IMAP, I can literally keep as much of my email as I want, and always get access to my full message history. On top of this, I have 7 inboxes to check.
