go ahead, edit it
I usually don't post little tips like this, but this one is just too simple, useful and undiscoverable to pass up. Courtesy of the 43 Folders discussion list: subject line editing in Outlook:
First, double click the e-mail (or hit enter) to open it in a separate window. Then put your cursor on the subject line and start typing. That's it. You can add your own reference - date, project number, whatever - or change the subject line completely. Whatever is going to be meaningful to you. Use as many words you like. When you're done, Ctrl+S to save.