Arrange your labels in Gmail as a hierarchy of folders.

 I've been trying to learn the ins-and-outs of Gmail's IMAP service over the last week or so.  One of the things I really like is the ability to replicate the Outlook folder structure with Gmail's label structure.

I've learned that dragging an email to a folder within Outlook, for example, won't create the label in Gmail.  You have to create those by hand.  This isn't so great, but the ability to "nest" labels is pretty cool.  Arend v. Reinersdorff has a great description of the process, including links to greasemonkey scripts to bring it all together.